Set part of forum open

At this moment i have hide my complete forum from Google (no index no follow) and only registered users.
Now i would like to set a (new) part of this forum open for everyone, it this possible ?

Sure. Setup the categories you want to remain private so that they require membership in a particular group (no permissions for “everyone”; note that trust_level_0 is "all users trust level 0 or above), and then remove the “login required” site setting. Categories which “everyone” can read will be publicly available, all others will still require login (or membership in another group, potentially) to be seen.


Does this mean that the other part of the forum (membership) also will not be indexed bij Google ?

That is correct. You can’t have a webpage that Google can see but ordinary (non-logged-in) web users can’t.