Broker: n. One that acts as an agent for others, as in negotiating contracts, purchases, or sales in return for a fee or commission.
Facilitator: facilitator works with a group of people to help them have a conversation, come to agreement, or plan for the future. In general the facilitator acts as a trusted and neutral outside voice, making decisions about the process the group goes through but allowing the group to focus on and control the content of the discussion. The facilitator is a gentle guide, making it easier for the group to have that discussion.
Go-Between: n. One who acts as an intermediary or messenger between two sides.
interface: A point at which independent systems or diverse groups interact: "the interface between crime and politics where much of our reality is to be found" (Jack Kroll).
Intermediary: n. In sales, the term intermediary describes how pulling buyers and sellers together while aggregating data can facilitate transactions. Internet-based intermediaries create multi-vendor, multi-product marketplaces.
Intermediator: n. A mediator.
Intervenor: To involve oneself in a situation so as to alter or hinder an action or development: "Every gardener faces choices about how and how much to intervene in nature's processes" (Dora Galitzki).
Mediator - n. One that mediates, especially one that reconciles differences between disputants.