why were they friends with Francis

I kept wondering why Tommy was friends with them. What sober person would want to hang out with a bunch of junkies?? None that I know of.

I grew up in a similar working-class community on a council estate and friendship groups like this weren't uncommon.

As other posts have pointed out, they were probably at school together, and those kinds of childhood friendships tend to be enduring, at least until you manage to escape from the area. Although in Renton's case, even moving to London wasn't enough to shake off Franco...

Begbie was clearly that group's "alpha" (or at least saw himself as such), and evidently had a long-standing reputation as a local hard man. So, guys like Renton, Sick Boy and Spud, having already fallen into Begbie's orbit, continued to hang out with him, probably for protection and to use him as muscle if/when needed.

Sure, Begbie bullied and dominated them, but there's no doubt that if anybody else messed with them, he'd be there to stick a glass in their face. In his own fucked-up way, Begbie did care about them.