I live in St. Cloud and have for most of the last 10 years, so let me know if you have more questions.
I don't know much about special needs groups, but Community Ed might be a place too start investigating there. CAtholic Charaties or United Way would both be places I would start if looking to volunteer with the homeless.
The St. Cloud times Sunday paper generally lists volunteer opportunities as well.
Are there other things you are interested in finding out about, St. Cloud can be amazingly like a small town and its sometimes hard to find community at first. The Good Earth Food co-op is one place I first met people when I moved here.