The OPMA is silent on allowing non-members, like consultants and focus groups, into an executive session. The WA Supreme Court has held that the exemptions to the OPMA must be construed narrowly (Tacoma v. Miller). My informal advice is that a non-member of the council may attend a closed executive session if their presence is vital to fulfilling the purpose of the executive session. For instance, if the executive session is to discuss pending litigation, then the attorney for the council is vital to discuss the litigation. If the purchase of real estate is the topic of an executive session, then the assessor's presence may be vital in order for the council to determine how the property was appraised.
I believe it is inappropriate to bring a "political" consultant into an executive session to discuss pending litigation. First, it is very arguable that by the mere presence of the consultant, the city has waived the attorney client privilege. Second, the authority to conduct an executive session for "pending litigation" is conditional. It may only be conducted if public knowledge of the discussion "is likely to result in an adverse legal or financial consequence" to the agency. RCW 42.30.110(1)(i). A political consultant under contract with the city is still a member of the public and therefore the presence of a consultant indicates that knowledge of the discussion would not create an adverse legal or financial consequence to the agency, and the closed meeting may be improper. Third, RCW 42.30.040 states that conditions on attendance of meetings are prohibited. Allowing consultants and focus groups to attend but not other members of the public may be a condition to attendance that would violate RCW 42.30.040.