The maximum number of users that you can add at one time to a group’s Add Users form is 1000. Looking at the code, it seems that you should get an error message similar to this if you attempt to add more:
“Maximum 1000 users can be added at once.”
There has to be some kind of limit, otherwise Discourse risks throwing an unhandled error. To get around the limit, I like the Automation Plugin approach that Jay suggested.