Hello @jyashi
Welcome to UEFN and Fortnite Creative! Here are the answers to your questions below:
Before UEFN was launched, creators would invite collaborators to their lobbies and enter a Fortnite Creative session this way. By doing this, creators were able to build and collaborate on maps together when given the proper permissions.
HOW TO COLLABORATE IN UEFN
If you are using UEFN and wanting to collaborate with creators now, you will have to add them to a team. Once the creators you add to your team pass verification, they will be able to join a UEFN session and help you build your map.
Get more details on how to collaborate and build teams in UEFN here
You are correct. At the time of this post - for UEFN team collaboration, the other members must also have UEFN. 
Hope that helps!
Dragon