Team matches must have even numbers of players to create the pairings.
Additionally, match organizers may set the number of players for each team in a match. Once that number of players is reached, other players may join, but may not be able to play.
The way it works is that all players on a team are rank-ordered by rating. Then the top players are chosen to play for each team. Players who are not in the top ranking cannot play.
Example:
Let’s say this is a 5 player match for each team. If 7 players from Team A join, only the top 5 rated will be allowed to play. The other 2 lower rated players would fall below the line (though they will remain as alternates until the match begins just in case the team admin decides to remove a player or one of the other players quits).