Party members join to maps.

I have a question for following situation.
I have alredy started to play a map. Then i see that one guy of my friendlist comes online. I invite him to party, but he is not able to join to my map.
Do we something wrong or how does this work?

If you invite him to a party he should be able to join your map.
But he wont be able to join your lab instance since its empty. You can fix that by either leaving the map so you are in the lab instance for him to join, or you can place a tp in the lab before you go into the map.

Or if you have found Zana you can invite her to your hideout and get the map device from her. After that you can start all your maps out of your hideout wich every of your party members can join even if its empty.

Thank you.