I have three people in my household that are all able to join. This is my setup:
I have a team created in creator portal.
They are all age verified and joined in to my team.
My island/project is under the team, not my personal.
I start session in uefn, they start fortnite normal. They join my party, then join my game in progress.
Once they are in, i am able to teleport them around via uefn, they can play as normal. If they are contributors or admin on the team, then they can make edits in creative 1.0 interface.
If you are setup exactly the same way, then i am not sure what could be wrong, but i can assure you i did not have to do anything soecial beyond the above description to make it work.