I also am having this issue and I'm playing Grounded 1. I create a shared world go into it everything is fine save. quit. Come back and the "Shared world" is greyed out. But my friend has no issue, if they start a game I can join no problem..... if they add me to their shared world it just never appears for me. Seems like this issue is still happening in Grounded 2 and I could not google fu my way out of this NO one has any fix and it appears the devs haven't even commented on it or suggested a reason why.