Hey there! I work on the UEFN documentation team, let me see if I can help you troubleshoot this issue.
Is your teammate trying to join you on UEFN in a joint session, or from Fortnite Creative?
For your teammate to join you from Fortnite Creative, you have to playtest the project. This way you will appear in your teammate’s Friends List in Fortnite. And as long as you are both on the same team the project belongs to, your teammate should be able to select you from the Friends List and click ‘JOIN’.
If they are trying to join you from UEFN and are being blocked, make sure the project belongs to the same team as you and your teammate. If the project does belong to the same team, then have your teammate restart their PC.
If problems still persist, let me know and I’ll escalate the issue to the UEFN engineering team. I hope this was helpful!