I currently have a custom game where players 1-4 (team 1)are on a team on one side of the the map and players 5-8 (team 2) are on a team on the other side. Player 4 is a computer allied with team 1 and player 8 is a computer allied with team 2. I have towers and units that are player 4s and player 8s.I have triggers allying all the players on team 1 with each other and all the players on team 2 with each other. I also have triggers to set the colors of each player.

I'm not sure how to make it so that player 4 and player 8 will always be set as the computer players, so when ppl randomly join the lobby they won't become player 4 or 8. I also don't need any starting locations, since it is a custom game with bases already built (unless they are needed for it to work).

Is it best to do this by triggers or game variants? And how would I go about accomplishing it by one of those means?

Thanks

Game Variants with Locked Alliance and Custom Team. Then go to Map Properties for Color, Controller and Start Location.
http://www.staredit.net/starcraft2/Player_ID

Sorry, but could I get a little more clarification? I know how to do it if it was users only who were involved, but player 4 and player 8 are computers. Its a 4v4 map, but only 6 users (3v3) should be able to join.
Do I set it as 3v3 and have some variant set player 4 and 8 as computers, so they will be computers on game load but not show in the lobby? Or do I have the computers locked in showing in the lobby as player 4 and 8?

How my map is set up, with the bases already built, player 4 and 8 need to be computers no matter what. The computers only have towers and units, while the users (players 1,2,3,5,6,7)have bases. If a player was accidentally put as what was designated as "player 4" or "player 8" in the editor, then that would create issues.

Hmm, those CPUs mind-fuck me.
An easy way to do this would be to make the lobby 1 team of 6 players, and in-game, make a custom interactive dialog box that would let players decide on which team they want to be.

Players 1,2,3 are right next to each other and players 5,6,7 are right next to eacho ther on the opposite side. As the game is (as a core elemtn of the game) there needs to be 3 ppl on one side allied, 3 ppl on the other, and computers as the towers and units

I need some way to essentially make the lobby like this:

Team 1
Player 1 - user
Player 2 - user
Player 3 - user
Player 4 - Computer (Locked)

Team 2
Player 5 - user
player 6 - user
player 7 - user
player 8 - computer (locked)

I got it working. I made player 4 and player 8 computers in player properties. under "(player) controller" i set 4 and 8 as computer and locked. For "(player) team [premade: 4v4]" i locked player 4 (as team 1 [4])and player 8 (as team 2 [4]).

Player 4 and player 8 are now computers no matter what, regardless of how/when people join the lobby.

Even if you allow players to move in the lobby to change of team, it will not do any changes. Watch out!