An agent can be defined as

Multiple Choice

parties on the same side that are working together and collectively advocating the same positions and interests.

one or more parties that have designated someone else to represent their positions and interests in a negotiation.

a negotiator representing the interests of another party.

any individual or group of people who are not directly involved in or affected by a negotiation, but who have a chance to observe and react to the ongoing events and who may be drawn into the negotiation.