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We already covered single user pickers, where you can change that field value based on the user who manually execute it:
Now it’s time for multiple user pickers, there are 2 options:
Set current user in this field on manual execution - on manual execution it will replace all users being mentioned in that field with the user who performs this action
Add current user to this field on manual execution - in this case, user executing it manually will be added to mentioned users