Hey.
I volunteer at a drop in centre for people who're homeless or vulnerably housed. A lot of people who use the service have MH stuff and substance abuse issues. I've been doing it on and off for about three years now, and I love it - I've met some great people and made some brilliant friends.
Only thing I will say is that it can be quite intense. Depending on what you're doing, you can hear some distressing things and have to manage some challenging behaviours. When I'm at my best I can do it no problem, but sometimes I just go to help set up the hall as I'm not up to dealing with it. You have to look after yourself and I'm lucky that the people who run the centre I work at are really understanding.
A couple of the other volunteers, including the person who runs the place, know about my MH history and admissions and they're fine with it. I was completely open on interview and they said it wasn't a problem and they've always had me back after a spell away. A lot of the other volunteers have had similar experiences or are in recovery from drugs and/or alcohol and are very accepting.
The do-it website is brilliant. Tap in your postcode and it'll tell you pretty much everything in your area.
Good luck.