From SmashWiki, the Super Smash Bros. wiki
A crew is a group of smashers who play Smash Bros. together, and often enter tournaments together. A crew can be of any size, though a crew of only two players is generally considered to be a team instead.
The purpose of a crew varies depending on the crew's own goals; in general, however, crews can allow smashers to form a sense of camaraderie in a region, and players in a crew often train with one another for tournaments. Crews can also allow for players to room with one another for tournaments, or transport each other to such events. Rivalries can also develop between crews, owing to regional differences, or a desire to simply be considered the stronger group, such as with machinima.
Crews should not be confused with sponsors. A crew is an unofficial group that supports players within its ranks; a sponsor, however, provides goods and services to players in order to represent them at events.