(This article does not apply to Indy Teams....for Indy Teams, Click Here)
For the regular season, you are allowed to bring players who are not on your roster. You'll just want to make sure that they sign the waiver before the game.
Additionally, in the regular season, you are able to pick players up from another team at the field and have them play with you. (Game officials may limit the number of pick ups to make sure that it still looks like your team out there or to make sure you're not picking up the best player in the league and it changes the competitiveness of the game).
Subs are not allowed in the playoffs, except in the case of injury. Those subs must have been approved by the office prior to your game.
A playoff sub can be requested here:
https://www.underdogseattle.com/page/Playoff-Substitute