Membership2pro - add existing users (multisite)

I'm unable to add existing (multisite/site-wide) users as Membership2pro users although they were added after M2 instalation.

The M2 searchbox can only find the admin (who is attached to the primary site in the network) but the other users who were added from the network admin users panel can't be found.

What's the best way to manually add new users so that the whole network can have protection and memberships?