I've been researching this for some time and have to believe I'm not the first one to ask/look for it.

My community has a set list of about 150 groups that never change.  People are only a part of one of these groups so it would be a dropdown list.  When they select that option, I'd like it to automatically assign them to a group which I could then grant permissions to view the various forums for that group.  

TIA for any help!

1. Peer to peer technical support or product guidance would be better forums on how to use the software.  Feedback is specifically ideas or suggestions to improve the software.  

2.  Do you mean 150 member groups? Or do you mean 150 choices on a profile custom field? 

If 150 member groups, then all you need to do is choose your group permissions for forums.  

If 150 choices on a profile custom field, then there is nothing.  Profile custom fields are meant for information, not to trigger new permissions.  You would need custom work.

3. Another option for you to think about: clubs.  This allows the members to pick and choose clubs to apply.