The user selector is used throughout Moodle to provide an interface for performing bulk actions on users such as:
- creating and managing groups
- managing forum subscriptions
- assigning roles
By default Moodle will only list users in the user selectors when you have less that 100 users on your site. This makes performing bulk actions on users where ever there is a user selector tedious.
To make Moodle show all users in the user selector, go to:
Site administration ► Users ► Permissions ► User policies
Increase the "Maximum users per page" setting to more than the number of users on your site.