It would be helpful if we could view a list of communities from instance detail tab, not a local view of threads like when you look at an instance from a guest account, but a list like when you search.
Spoiler
This is planned; we're tracking the issue here.
It's possible to view this list in the search tab on the Mlem beta. To do so, open the search bar and type a space, then tap on the filter button (just below the cancel button) and choose an instance to filter for.