A user of IBM Connections 4.5 must be member of the parent community before he/she can become member of a subcommunity.
Adding a member with role "owner" lets the new member be a Community owner (so that multiple users can be owner of a community).
I have found that adding a member to the parent community "as owner" will force ownership for all parent's subcommunities to this member (at least as far as this user is member of the subcommunity).
Why is that?
I asked the architects responsible for Communities.
There is a reason for this which is that that a Owner at a top-level, means that the person has the full-lifecycle of operations available to them, including setting owner on any subcommunity. Adding them to the subcommunities automatically is so that there are no-orphaned communities.
I hope this helps.