There is matchmaking and its called Guided Games.

Do you need a Fireteam for Leviathan?

Youll need a full Fireteam of 6 to take on the Leviathan Raid. Now, the Fireteam leader will add you, through either PSN or Xbox Live, and youll have to accept their friend request, before joining their in-game Fireteam.

Can you 4 man the Leviathan raid?

The Destiny 2 Leviathan raid has now been beaten by a four-person squad of players. Unsurprisingly, the impressive feat was completed by a clan called The Legend Himself, which was also the worlds first group of players to successfully complete the raid.