1. A method in an initiating endpoint having a memory and a processor for establishing a multiparty session that has not yet been established with a full-mesh configuration, the method comprising: sending, by the initiating endpoint, in parallel to each of a plurality of endpoints a first Session Initiation Protocol (SIP) invitation, wherein the first SIP invitation indicates that a parallel invitation protocol is being used to establish at least a portion of the multiparty session that has not yet been established and that the invited endpoints must support the parallel invitation protocol in order to accent the first SIP invitation, wherein an endpoint that supports the SIP parallel invitation protocol must also support the SIP serial invitation protocol, and wherein sending first SIP invitations in parallel includes sending the first SIP invitation to a first invited endpoint and, without waiting for a response from the first invited endpoint, sending the first SIP invitation to a second invited endpoint; each time the initiating endpoint receives a SIP response indicating an acceptance of the invitation sent in parallel from an accepting invited endpoint, sending by the initiating endpoint to the accepting invited endpoint an indication of the invited endpoints that had already accented the first SIP invitation at the time the SIP response indicating acceptance was received, wherein an endpoint that supports the parallel invitation protocol, upon receiving the indication of the invited endpoints that have already accented the first SIP invitation, sends SIP invitations to each of the indicated endpoints to establish a dialog of the multiparty session; and after receiving, by the initiating endpoint, a rejection from an invited endpoint in response to the first SIP invitation, wherein the rejection indicates that the invited endpoint does not support the parallel invitation protocol, and wherein the rejection does not indicate that the invited endpoint does not want to participate in the multiparty session, sending, in serial, by the initiating endpoint, to the invited endpoint that rejected the first SIP invitation a second SIP invitation, wherein the second SIP invitation indicates that a serial invitation protocol is being used to establish at least a portion of the multiparty session that has not yet been established, and includes an indication of the invited endpoints that have already accepted the first SIP invitation at the time the SIP response indicating acceptance was received, wherein for each second SIP invitation that is sent to an endpoint that rejected the first SIP invitation the initiating endpoint must wait for a reply to the second SIP invitation before sending a second SIP invitation to the next invited endpoint that rejected the first SIP invitation, and wherein an endpoint that accents the second SIP invitation, upon receiving the indication of the invited endpoints that have already accepted the first SIP invitation, sends SIP invitations to each of the indicated endpoints to establish a dialog of the multiparty session.