Sly Stone's former bandmates tell TMZ ... they were SHOCKED to learn the 68-year-old singer is homeless and using drugs ... but they wanna help the guy ... by inviting him back into the band.
TMZ spoke with Greg Errico -- the band's original drummer -- who tells us he hasn't spoken to Sly since last year when the band reunited for Coachella’s 2010 Festival -- and Sly bombed.
But Errico says the band considers Sly a true family member -- and says, "He knows if he ever wants to make music with us again, the door is always open.”
Greg tells us the entire band is clean and sober -- and believes Sly could benefit from being around positive influences.
As we previously reported, Sly -- who admitted he has problems with cocaine -- has been living in a van in L.A. ... but insists he wants to go to rehab to clean himself up.