We at Styleware made a few modification in order to allow auto assign of K2 group to newly registered users, even if the process goes to another extension (i.e. JomSocial).The handling of user assignment was very disturbing in terms of users not being able to comment or add entries, just because the registration was not through the K2 form (fields).

Therefore we made small modification to the K2 User Plugin to allow auto-assign even if the registration goes through JomSocial, VirtueMart or anything.

Check out the modified Synchronization K2 User Plugin. Please read the description carefully before using.

Exactly the problem I have - I had to develop my own reg. form and of course no user group assignment for K2 users...

Gonna give your plug in a try and get back to you.