Some other seiyuu groups have no relation to any anime or games, and they just perform together for songs or events.
The following page has a large table listing many of the known major (and even very minor) seiyuu groups and their members.
Some of the most popular (or interesting) current groups are as follows.
search the groups database
The search string can be anything, like a seiyuu name, show name, etc..
Some regular expressions will work, and all searches will be done in a
case insensitve manner.
When typing in names, use the "LASTNAME FIRSTNAME" order.