If your friend or a member of your crew/clan is in a match, you can spectate them by:

1. From the left side of the lobby, select the "▶(friend)" icon
2. Select the category from "Friend", "Crew" or "Clan" and press the ▶ button.

If you don't want others to spectate your matches, you can go to [Settings -> General] to turn on/off spectating