I thought I could get it to generate a field in the registration form to get the member to choose a group to auto join, but I couldn't figure it.

You can, just navigate to CB > Field Management and create a new field of the type "Auto Join".

so I added a new field asking them to select which branch they belong - this i sok, but I want this so I could auto join them to one of the fixed groups I've defined. but I just don't get all those triggers...onBeforeUserProfileEditDisplay,onStartSaveUserRegistration.. etc... then the conditional !?!?
help? nothin in the user guide on how to user this...

That's more advanced then using the auto join field. I don't recommend this approach if you've no experience with triggers or substitution usage. If you'd like to learn more on triggers and substitutions then please see the below. Please note the drop-down next to triggers makes it easy for you to quickly select where you want the auto join to take affect. Please also read the notes to the right of every input as they explain what the input does.

Trigger information: www.allmysocials.com/directory/tutorials/item/228-event-triggers
Substitution information: www.allmysocials.com/directory/tutorials/item/233-substitution-usage

Conditions support substitutions so you could do "[username] Equal To Bob" for example. This would mean the users username must be Bob for the auto to take place.