Several multimedia applications like video-conferencing and distributed interactive simulations require efficient support for multipoint-to-multipoint connections from the underlying networks. The routing and signaling protocols for supporting these connections in ATM networks have been proposed earlier by Venkateswaran et al. (see IEEE ATM'97 Workshop, p.263-71, 1997). In this paper, we address the sender identification problem which occurs when multiple senders send data to a multipoint-to-multipoint connection that is identified by a single connection identifier. The various solutions to this problem are systematically categorized into two categories. Using simulation studies of the representative techniques for the two categories, namely VP-merge and VC-merge, we motivate the acceptance for the VP-merge mechanism by showing that the VC-merge technique may require up to 53% more buffer than VP-merge for some typical scenarios. We propose solutions to improve the scalability of the VP-merge technique and describe details of its implementation. Specifically, we identify the VCI collision problem associated with the VP-merge mechanism and propose and analyze different solutions to prevent or resolve VCI collision. The VCI collision resolution mechanism also introduces VP-VC switching, a new mode of operation for the ATM switch, that is different from the standard VP-switching and VC-switching modes of operation.