Originally posted by SubsidedLemon:

Do you use, or did you use mods?
Do your friends use mods?

Did you try joining friends by IP using the ingame option? (they have a dedicated server?)

No mods, neither me or they. I did not try to join by IP since none of us know how to check the IP and since I see no reason why it would work. We would be playing in one of my friends world. My other friends have no problems joining, but for some reason I could not join them. Thank you for the suggestion though.