When I first started playing I had no issues with seeing my friends that play ESO in the ingame friends list. It seems since I joined a second guild that this list is now blank. Not sure if this has anything to do with it but it just seems odd that they vanished after joining a second guild.
Sometimes they switch friends from list to guild roster, so you have to user roster to locate friends in same guild. If you delete guild friends may become lost. I experienced this but I'd rather be able to login to the server which I cannot at this point.