Apart from directly participating in a custom match, players can join in as spectators. It allows them to watch the game from any player's perspective.
To join as a spectator, you will first need to know the Matchmaking ID the host used to create the match. Once you get it, go to the Menu - Settings - Developer and select Spectator Mode. Enter the ID there and tap Confirm. The queue will start immediately, and you will join the game in a few seconds.