Each DX200 base canhave 15 users registered to it and this gives you 4 talk paths to be sharedamong the 15 users. All 15 can hear the conversation but only 4 can talk atonce and this is based upon a first come first served basis within the 15 usergroup. You can add a 2^nd base and use a 3 pin XLR cable to pass theaudio between the two bases.  However you can only add the additional 15users to the second base.  All 30 users will hear each other through theXLR cable attached to the 2-wire port on both bases. You can also use the4-wire ports to connect the audio between the bases using RJ45 Cat5 cableconfigured as an audio cross over cable. A standard Ethernet cable will notwork.