Not sure what is the exact issue for users.
We will be happy if you can tell us more, how to replicate it or record quick video.
The product team will be interested.
It might help to disable oin-line edit option. (let me know if you like and I can point you to how to make it. It might resolve your issue (if its really in line edit stuff)
but back to the question,
There is no bulk operation for moving users, but it should not be too difficult as the membership is on group level. So you open group, and select all members, one by one.. the same with removing. I belive its pretty quick, as you dont need to move out of group object or save after each entry. For new users you can set the "defaultGroup" to readers and they will became readers by default.
Just good to know if you move users to readers, they will not be able to contribute to content.
Hope it help at least a bit.