I’m not a Bee Gees superfan so I’m not totally certain when this group photo was taken. But based on the little I do know about the group and what they look like I’d put this at around 1967-68, give or take a year.

If I’m correct that means the band lineup was Barry Gibb, Robin Gibb, Maurice Gibb, Colin Petersen, and Vince Melouney. Melouney was the band’s lead guitarist for a brief time and left in 1968.