I have to share a history with my class. I add the first set of 17 but then need to add a few more. Once I add the first 17 then I can see the list of users who now have access but when I click on the "Share with another user" button nothing happens. I can go back and delete all the users with whom I have shared the history and then I can add new users. That is a pain because I would like to add shared users as the students get their accounts created.
I can reproduce this at Galaxy Main https://usegalaxy.org.
There is probably a bug with the function and I have opened a ticket for the developers to review/address the issue, please follow it for updates: https://github.com/galaxyproject/galaxy/issues/5783
For now, keep a comma-separated list of the student emails, add to that when new accounts are created, then clear/replace all in Galaxy when you need to add more. (What it sounds like you are doing).
Thanks for reporting the problem! Jen, Galaxy team