Using CB Auto Actions and the AcyMailing action you can subscribe them to a newsletter when they join a group. This would be done for example using the below usage.
Group Join:
Global
Triggers: gj_onAfterJoinGroup
Type: Usergroups
User: Automatic
Access: Everybody
Conditions
1: [var2_id] Equal To GROUP_ID_HERE
Group Leave:
Global
Triggers: gj_onAfterLeaveGroup
Type: Usergroups
User: Automatic
Access: Everybody
Conditions
1: [var2_id] Equal To GROUP_ID_HERE
If you want it to act on any group join simply remove the conditions.
If you want a CB userlist with all users in a specific group that's doable using the Advanced filter within the usergroup and simply querying for it. Example advanced filter as follows.
Code:
( SELECT COUNT(*) FROM `#__groupjive_users` AS gu WHERE gu.`user_id` = u.`id` AND gu.`status` >= 1 AND gu.`group` = GROUP_ID_HERE ) >= 1