Currently Fortnite Battle Royale only offers Solo play, but Squads are on the way.
You can Squad up with your friends as of the 1.6.3 update, which launched on 09/26/2017. The Squad match type (up to four-player teams) will be available to play immediately after the update. Duos (two-player teams) will be activated when there are enough players to support matchmaking in both modes.
To read more about Squads and other details regarding this update, check out the 1.6.3 Release Notes!
As always, ad hoc teaming in Solo mode is a violation of our Code of Conduct and may result in bans for players who are identified as having done so.