You can declare any player you meet in the game to be a teammate. This instructs the game not to perform ambush fire on a teammate passing through your worlds. This is all it means!

Note that declaring someone your teammate doesn't necessarily mean that you trust them or even that you intend to work with them. All it does is disable the automatic ambush fire against that player, until you declare them no longer a teammate.

The other player will not be told that you have declared them your teammate. Note that there's no reason to assume that the other player has also declared you a teammate!

In the RSW Classic game, there is a further distinction between allies and loaders. There is no such distinction in the Core game.