GroupFinder is an extension to the MatchMaker to allow premades to be formed in an easier way.
The addon hooks up to the existing MatchMaker so no new window to handle.
It allows you to create a premade group with any of the group-content in mind.
Dungeon, Adventure, Battleground, Arena etc.
You may specify which roles you would like to fill and adding a message to your listing about whatever you are looking for etc.
You can watch all premade listings and choose to communicate with the creator of the premade
or click the join button which will send a Join to the leader of the group, which he may accept or refuse.
You can find the official thread on the forums: here
If you are having errors first check if there are two instances of GroupFinder in your Addons menu.
If there are, follow these instructions: here