Mapmixer has 8 slots for 8 players. when you load the map only the players that are available will be modifiable.
if a map have 5 players - slots 1 to 5 will be modifiable.
If you set:
- slot 1, 2 and 3 to value 1, this will make the fist 3 players to be in Team 1
- slot 4 and 5 to value 2, this will make the last 2 players to be in Team 2
Then apply the change and check in game.