Assignee is a user assignment field on Requirements, Scripts, Script Assignments and Incidents. This field is synonymous with the AssignedTo field, and is provided as convenience for JQL users.
To find all Script Assignments assigned to multiple users:
Assignee IN [username1, username2, username3]
To find all Script Assignments assigned to members of a group ( e.g. Testers):
EntityType = ScriptAssignment AND Assignee IN MembersOf('Testers')
=, IN, !=, NOT IN, IS, IS NOT