The Age-group lead coaches have the "Commissioner" role in teamsnap for a specific division. It allows you to add players and modify rosters, as well as assign team managers.
The first thing you will want to do is add yourself as a team manager for the teams that you coach directly: add division commissioner as a team manager
Secondly, you will want to add assistant coaches and non-commissioner coaches as team managers
Finally, you or the team's head coach should review the roster and invite the team to join TeamSnap
Returning Commissioners should archive old teams before inviting players
Commissioners have the ability to move players between rosters
To put a player on more than one roster (eg, as a guest) Commisioners can find the player in the "Members" tab, then select "Actions->Copy Profile" for the player. This copies the player onto a second team. For players that rarely act as guests, it's better to just manage their attendance via email.