Hey Boris,
Can you check my following point and see whether i understand correctly with your requirement?
1. Whoever Group member post a discussion in different type of group (open/closed/invite), you would like non-member can be accessible these discussion post but only group member can reply?
Or
2. Whoever Group member post a discussion in different type of group (open/closed/invite), you would like non-member can be accessible these discussion post and everyone also can reply?
And when you talk about the privacy, you mean that in terms of database, they are separated? Fórum discuss/easydiscuss?
Perhaps you can look at my following explanation, i believe you can understand more about the default workflow.
1. As long as that discussion posted from group, only registered user can able to access.
2. Open group - Registered member only can access
3. Closed group - Only group member can access
4. Invite only group - Only group member can access
5. All these group discussion only appear on Easydiscuss > group page