When I call fort_team_collection.AddToTeam, will the target agent get moved to the InTeam team - or - will it fail?
( I wish I could test with more than player on my own 
)
What I really need to know is if said agent is already on a team then will that agent get moved to the new team or can an agent exist on multiple teams?
It isn’t exactly explicit in the documentation and I haven’t set up an Epic Team yet to test my code with more then 1 player.
No you can only have one team, and actually the agent doesn’t move, it just changes its team value
Also, it can be a good idea to get another device to test your maps, I believe Fortnite runs on many consoles and on Android, also having a cloud computing subscription can be a good thing 
Not mandatory but it can help 