Upon motion of any party, the ALJ must order witnesses excluded from the hearing room so that they cannot hear the testimony of other witnesses. This rule does not authorize exclusion of—

(a) A party who is an individual;

(b) In the case of a party that is not an individual, an officer or employee of the party appearing for the entity pro se or designated by the party's representative; or

(c) An individual whose presence is shown by a party to be essential to the presentation of its case, including an individual employed by the Government engaged in assisting the representative for the Government.