Squads unlock once you reach the A-List. People who have done comebacks will also be able to join squads. If you drop below the A-List, you can continue to use Squads. If you’re not at the A-list but you’ve done a comeback, you can still join a squad.
Once you’ve reached the A-List, the Squads button in the bottom right will unlock. Then you can pick from the list of recommended squads or search for a specific squad to join.