If you've too many groups it stops displaying them as options to select. Instead you'll have to add the groups manually via their group id. This prevents the page from crashing due to too many select options. Eventually this will support ajax searching behavior so you won't have to manually supply group ids when you hit the limit, but for now that's unfortunately how it must work. Since it's a tag field you should be able to just type the group id then press enter to add it.