Overwatch doesn’t really seem to have a spectator mode or a reliable way for you to spectate other players’ games just yet, but here’s are a couple of ways you can do so. If you join a friend’s game, but that game happens to be full up, you’ll end up in spectator mode where you can observe your friend’s play style, along with all the other players in that current game. If your friend chooses to stay in that group matchup, and the games continue with no vacancies, you’ll stay in spectator mode as well.
If you join a game on your own, and all team spots happen to be filled, that will put you in spectator mode too. However, do take note that once a spot is freed up, you’ll automatically be placed into the game. There doesn’t seem to be a way to back out of playing and just remain as a spectator for now, unfortunately. So if you want to spectate multiple games in a row, joining a friend’s group in a full match seems to be the most reliable way of doing so.
Blizzard might eventually update the game with a spectator mode, and we’ll keep you updated once there’s a better way of spectating matches in Overwatch.