You want to add a link to their GJ group to the CB Invites email or to the GJ invite email? You can't add it to CB Invites as CB Invites has no idea what groups they belong to and if it's more then 1 it would need to know how to behave (does it add them all? add latest? add last? etc..). For GJ invites you can add the [group] substitution to the invite message.