How do you actually do that, out of interest?

How to prevent User from reading/writing to a Forum of a Groups he is not a Member off?

What do you mean exactly? I set just the displayorder to 0.

Which version are you running? If 1.3, just check permissions for his usergroup.

Sorry, then i don?t understand your Descriptions in the AdminCP for the Permissions:

There are only the Options to read or write in public or non-public Forums.
There is no Option to access/show only Socialforums of Groups where the Member is subscribed to.

So what did i have to allow/deny for this?

And at least a Featurerequest:
[*] Option to delete all old Messages of a Group if a Socialforum is created from Database

You asked in your first post for "Groups he is not a Member off", now for "where the Member is subscribed to"...

If you are asking for showing the forum, it's included in the read-permission. If the user can read in the forum he can also see the forum.

Awesome!! Now this add-on is perfect!!

Nominating you for mod of the month!

Here is the best way to make this look/feel the best:

1. In the Socialforums settings "Visibility"- Set to NO.

2. Make a Forum Category Called "Social Forums" (Set Active Status- NO) It will be hidden.

3. Make a Forum Called "Social Forums Home" (Set Active Status-NO) It will be hidden. This is the Forum where the Social Forums get "subbed" to. (This is the Forum ID number you will need to set in the Socialforum settings.) They will all be hidden from view from EVERYONE, you included.

This makes Social Groups more private and doesn't clutter up the Forum Home.

Will they show in new posts still with your settings Baldilocks, or only if they have read/write permissions?

You can set this behaviour via usergroup permissions. Set to No for all Groups would only members allow to see the forum, it's posts, etc...

They will only show if they have read/write permissions.
For added bonus set the Main Forum (the ID number for socialforums add-on) to not be indexed in searches either.

Even I as Super Admin can't get into the "Ladies Only" social forum--I already tried, nor do their messages show up on a New Posts/Today's Posts search.