Thanks very much for the prompt reply, @callmevlad!
Unfortunately, what you describe as flexibility seems to me to be unnecessary work. I would indeed want to list groups per member as well as vice versa. It's unclear to me what benefit there would be to manually managing two separate one-to-many relationships every time a member joins or leaves a group.
I did try a separate "cross reference" collection which had two Reference type fields - one for members and one for groups. That's similar to the way it would be done traditionally. However, there's no way to "look up" or pull data from the referenced collections.
Many-to-many relationships are very commonplace in the real world, and WF's lack of support for them, unfortunately, precludes its use for projects I have in mind. Another example would be photographers and photo albums. Let's say a photographer can create albums for categorizing their work. An album can, of course, contain many images, and an image should be able to reside in more than one album - e.g. a bee on a flower might be in both the Insects and Flowers album.
Unless support for this is right around the corner, WF just won't work for my projects. I was willing to try to proceed with the lack of support for mobile-first development, but a lack of many-to-many relationships is a real deal breaker.