There are many different ways to change teams or select a certain team so i made different ways to show you this.
Here you join a game and its 2 teams but you want to be on Team Alpha, you click that team and you join it, now there are some rules to it, if the team has too many players it wont allow you to change to their team, like if 2 players are on a team and you are on the other by yourself you cant join their team.
Now there is changing teams in the country select stage. Here its simpler, just select Join Team function to join the other team, unless like the problem b4 its too full.
The 3rd and final way to select a team is what Clovis talked about, selecting a scenario faction that is based on a team. For this I used ultimate ww2. Here in this map whichever faction or country you want is put on a team determined by the person who made the scenario. This is the final example of how you can select/change teams in atwar.