We commonly need to add the same 12-15 reviewers to our pull requests. There are links to the ten most recently selected, which are easy to use. But then we have to go back to our notes or memory to remember who else to add.
When adding a reviewer by typing part of the name, the selections offered include an image saying, "TEAMMATE". This suggests that the system has a concept of the folks that I would like to add. I would enjoy a button for adding an entire group. Or perhaps a group of users with checkboxes.