At the time of writing, when users (with Manage Watchers permission) try to add watchers to an issue, the user can only add one user at a time. To add multiple watchers, the process involves repeatedly:

• clicking on the "+ Add watchers"
• typing in the watcher's name for auto-complete to suggest the user
• clicking on the user to select and add it to the watchers list

It would be great if the watcher selection behavior can be simplified similar to populating a multi-user picker field, where we can search and select all the desired watchers and add them all in one go (example screenshot below):