Hot answers tagged scaling
Here is my solution for such a problem. Edit one group at a time. When you need to add more members, click to load overlay window which lists everybody but also shows you who has been added to the group already.
You could use that shuttle design you mentioned but to get around the scrolling issue, include a search box on the left side above the members list. This way, if you know the names of the people you want to add, you just find their name, press Enter, and move on. If the users are already in groups, you could show those groups in the left side list and ...
Only top voted, non community-wiki answers of a minimum length are eligible