You can configure standard user-picker fields to follow similar rules as the assignee field does. Create one and have a look at the configuration after its been created - you should find a way to set up some rules.
when I go to the following
Find the custom field you want to configure and click > Configure > Edit Options > Edit User Filtering.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.
It must be a user picker, not a select list or other type of field.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.