USUALLY, you can search in the box at the top right that says "Search Social Network".
However, Ning recently experienced a problem searching networks that include a "-" in their name -- like My.gain-net
We have reported the problem and they are working diligently to fix it. When fixed, it works great -- I've used it multiple times to discover new friends. Until that bug is fixed, the more you participate in discussions the more people will find you ;-)