I'm looking for a plugin that would allow me to create private groups for discussion that are invite only. These would be for people who have taken a workshop and want to remain in communication with their cohort afterwards. They would need to be invited/added by admin after each workshop and would not communicate with other participants. Anyone have any ideas? Appreciate the help!
You can easily create private forums with the Simple:Press forum plugin.
It can do a LOT more than just that, and for that reason might be overkill (not sure what other requirements you have), but if you are looking for a very flexible and powerful forum system, then I can recommend Simple:Press.
BuddyPress is good for this as well. You can create public, private or hidden groups and control who can invite others.
http://wordpress.org/extend/plugins/buddypress/ You have the option to have forums in each of the groups as well.