They were definitely a group. A group that evolved out of the Wild Bunch, which was a kinda collective sound system thing in Bristol.
They just worked with guest performers who had higher personal profiles. Even Tricky falls into that category. Let alone Sinead O'Connor, Tracey Thorne and Liz Fraser.
The original Massive Attack was 3D (Robert Del Naja), Daddy G (Grant Marshall) and Mushroom (Andrew Vowles). Mushroom left after "Mezzanine", apparently he didn't like it! A guy called Neil Davidge replaced Mushroom. He'd already worked with the band on "Mezzanine".
Daddy G then kinda left, and kinda didn't. He certainly didn't contribute to the recording of "100th Window", but he did appear with the band live in concerts and at festivals after it was released. Apparently, however, the Massive Attack that is supposed to be recording a new album consists of 3D, Neil Davidge, and a computer programmer called Swift!
Oh, incidentally, Massive Attack is one of the best live shows I've ever been to.