Hi drew,
The way to join an alliance is to look around your area and see if other players are in an alliance and if so, send them a message and ask for an invite to join their alliance.
Another way is to look through the list of alliances is to click the top tab where it says "Ranking", then click the "Alliances" tab. This will show you the list of alliances and their ranking on the server. Clicking on any of those alliances will pop up a box showing details of that group, including a list of players in the alliance. Once again, just message one or two of those players and ask if you can have an invite to join their group or alliance.
Also for future reference, the EA staff & developers for this game have previously stated they do not monitor or follow these forums. Also, very few experienced players of this game (if any) come here too. You would do better posting all future problems/comments regarding Tiberium Alliances to the dedicated C&C:TA forums at: http://forum.alliances.commandandconquer.com/index.php
