This JQL function can be used to search for users by checking against user properties that contain text values.
usersWithTextProperty(property, operator, value) #Output: User list
If you want to search for issues where a user as a specific numeric or date property value, have a look the the usersWithNumericProperty() or usersWithDateTimeProperty() function.
If you want to search for issues, checking only if a property is being set ignoring it's value, have a look the the usersWithProperty() function.