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.
See https://confluence.atlassian.com/adminjiracloud/configuring-a-custom-field-776636423.html#Configuringacustomfield-Howtofilterusersforuserpickerfields
when I go to the following
Find the custom field you want to configure and click > Configure > Edit Options > Edit User Filtering.
It must be a user picker, not a select list or other type of field.
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