My idea is:

• Make account icon a colorized to indicate a friend is a player / spectator in the lobby.
• Add a battle score to rank the lobby higher depending on how many friends are in.
• Add a filter to the lobby list to only show lobbies with friends.

Indication of friends needs to use a widely recognized "friend" icon, like would be found on other games or social media. The problem with using colours is that the user only knows what they mean if they already know what they mean, as well as being inaccessible to colourblind users.