Can I make a auto join a particular group when a user joins with cb connects?
What would be the code?

You don't add the entire block of code. I provided that so you could see what variables are available. Just add familyplans_onBeforeAcceptSharedSub to the triggers parameter.

I want the new (shared member) to auto join the parent users group jive group (MY GROUP) when they accept the invitation
We have to make this auto action?

Yes, the familyplans_onBeforeAcceptSharedSub is used to add custom behavior for when a shared subscription is accepted. What you're trying to do does not exist out of the box which means you have to implement it using CB Auto Actions.

For familyplans_onBeforeAcceptSharedSub var1 is the family share object, var2 is the user accepting the shared subscription, and var3 is the subscription being accepted.