Groups are currently limited to fifteen members. Everyone who has been invited to a group message can send and read all messages in the group, even if everyone doesn't follow each other.
The group moderator can add and remove other group members, but everyone always has the ability to leave on their own.
Group moderators cannot add people to the group who have blocked them. If you have been added to a group with someone you've blocked, you'll see a warning that someone you've blocked is in the thread with an option to remove yourself.