I was looking around everywhere and I couldn't find anything on allowing users to join or sign in under their Steam name.
Depending on the Steam API it may or may not be trivial to add to Social Login http://www.elkarte.net/community/index.php?topic=4370
HybridAuth supports Steam: https://hybridauth.github.io/hybridauth/userguide/IDProvider_info_Steam.html
Thank you both very much, but I know nothing about coding or how to setup.
@Frenzie this would be the best idea, to have it as an option in the social login.