Background: Pink Floyd were an English rock band formed in London in 1965. They achieved international acclaim with their progressive and psychedelic music. Distinguished by their use of philosophical lyrics, sonic experimentation, extended compositions, and elaborate live shows, they are one of the most commercially successful and influential groups in popular music history. Pink Floyd were founded by students Syd Barrett on guitar and lead vocals, Nick Mason on drums, Roger Waters on bass and vocals, and Richard Wright on keyboards and vocals.
Context: In December 1967, the group added guitarist David Gilmour as the fifth member of Pink Floyd. Gilmour already knew Barrett, having studied with him at Cambridge Tech in the early 1960s. The two had performed at lunchtimes together with guitars and harmonicas, and later hitch-hiked and busked their way around the south of France. In 1965, while a member of Joker's Wild, Gilmour had watched the Tea Set. Morrison's assistant, Steve O'Rourke, set Gilmour up in a room at O'Rourke's house with a salary of PS30 per week (equivalent to PS500 in 2016), and in January 1968, Blackhill Enterprises announced Gilmour as the band's newest member; the second guitarist and its fifth member, the band intending to continue with Barrett as a nonperforming songwriter. Jenner commented: "The idea was that Dave would ... cover for [Barrett's] eccentricities and when that got to be not workable, Syd was just going to write. Just to try to keep him involved". In an expression of his frustration, Barrett, who was expected to write additional hit singles to follow up "Arnold Layne" and "See Emily Play", instead introduced "Have You Got It Yet?" to the band, intentionally changing the structure on each performance so as to make the song impossible to follow and learn. In a January 1968 photo-shoot of the five-man Pink Floyd, the photographs show Barrett looking detached from the others, staring into the distance.  Working with Barrett eventually proved too difficult, and matters came to a conclusion in January while en route to a performance in Southampton when a band member asked if they should collect Barrett. According to Gilmour, the answer was "Nah, let's not bother", signalling the end of Barrett's tenure with Pink Floyd. Waters later admitted, "He was our friend, but most of the time we now wanted to strangle him". In early March 1968, Pink Floyd met with business partners Jenner and King to discuss the band's future; Barrett agreed to leave.  Jenner and King believed Barrett to be the creative genius of the band, and decided to represent him and end their relationship with Pink Floyd. Morrison then sold his business to NEMS Enterprises, and O'Rourke became the band's personal manager. Blackhill announced Barrett's departure on 6 April 1968. After Barrett's departure, the burden of lyrical composition and creative direction fell mostly on Waters. Initially, Gilmour mimed to Barrett's voice on the group's European TV appearances; however, while playing on the university circuit, they avoided Barrett songs in favour of Waters and Wright material such as "It Would Be So Nice" and "Careful with That Axe, Eugene".  Pink Floyd recorded The Dark Side of the Moon between May 1972 and January 1973, with EMI staff engineer Alan Parsons at Abbey Road. The title is an allusion to lunacy rather than astronomy. The band had composed and refined the material on Dark Side while touring the UK, Japan, North America and Europe. Producer Chris Thomas assisted Parsons. Hipgnosis designed the album's packaging, which included George Hardie's iconic refracting prism design on the cover. Thorgerson's Dark Side album cover features a beam of white light, representing unity, passing through a prism, which represents society. The resulting refracted beam of coloured light symbolises unity diffracted, leaving an absence of unity. Waters is the sole author of the album's lyrics.  Released in March 1973, the LP became an instant chart success in the UK and throughout Western Europe, earning an enthusiastic response from critics. Each member of Pink Floyd except Wright boycotted the press release of The Dark Side of the Moon because a quadraphonic mix had not yet been completed, and they felt presenting the album through a poor-quality stereo PA system was insufficient. Melody Maker's Roy Hollingworth described side one as "utterly confused ... [and] difficult to follow", but praised side two, writing: "The songs, the sounds ... [and] the rhythms were solid ... [the] saxophone hit the air, the band rocked and rolled". Rolling Stone's Loyd Grossman described it as "a fine album with a textural and conceptual richness that not only invites, but demands involvement."  Throughout March 1973, The Dark Side of the Moon featured as part of Pink Floyd's US tour. The album is one of the most commercially successful rock albums of all time; a US number 1, it remained on the Billboard chart for more than fourteen years, selling more than 45 million copies worldwide. In Britain, the album peaked at number 2, spending 364 weeks on the UK chart. Dark Side is the world's third best-selling album, and the twenty-first best-selling album of all time in the US.  The success of the album brought enormous wealth to the members of Pink Floyd. Waters and Wright bought large country houses while Mason became a collector of expensive cars. Disenchanted with their US record company, Capitol Records, Pink Floyd and O'Rourke negotiated a new contract with Columbia Records, who gave them a reported advance of $1,000,000 (US$4,962,213 in 2017 dollars). In Europe, they continued to be represented by Harvest Records.  After a tour of the UK performing Dark Side, Pink Floyd returned to the studio in January 1975 and began work on their ninth studio album, Wish You Were Here. Parsons declined an offer to continue working with them, becoming successful in his own right with the Alan Parsons Project, and so the band turned to Brian Humphries. Initially, they found it difficult to compose new material; the success of The Dark Side of the Moon had left Pink Floyd physically and emotionally drained. Wright later described these early sessions as "falling within a difficult period" and Waters found them "tortuous". Gilmour was more interested in improving the band's existing material. Mason's failing marriage left him in a general malaise and with a sense of apathy, both of which interfered with his drumming.  Despite the lack of creative direction, Waters began to visualise a new concept after several weeks. During 1974, Pink Floyd had sketched out three original compositions and had performed them at a series of concerts in Europe. These compositions became the starting point for a new album whose opening four-note guitar phrase, composed purely by chance by Gilmour, reminded Waters of Barrett. The songs provided a fitting summary of the rise and fall of their former bandmate. Waters commented: "Because I wanted to get as close as possible to what I felt ... [that] indefinable, inevitable melancholy about the disappearance of Syd."  While Pink Floyd were working on the album, Barrett made an impromptu visit to the studio, during which Thorgerson recalled that he "sat round and talked for a bit, but he wasn't really there." He had changed significantly in appearance, so much so that the band did not initially recognise him. Waters was reportedly deeply upset by the experience. Most of Wish You Were Here premiered on 5 July 1975, at an open-air music festival at Knebworth. Released in September, it reached number one in both the UK and the US.
Question: Who was in the band during that time period?

Answer:
In December 1967, the group added guitarist David Gilmour as the fifth member of Pink Floyd.