Only 4-bit grayscale windows at either 320x240 or 160x120 are used for pictures of conference participants. So, if you don't have any video equipment, you can join in a conference in receiver mode with just your Amiga and a modem. The approach CU-SeeMe takes is to make use of something called a Reflector. At the time of writing, Reflectors are only available on Unix systems. When you start CU-SeeMe, you are asked for the name of a Reflector to connect to.
Reflectors are a sort of conference server.
There are lots of Reflectors on the Internet, mostly running in universities or research labs. Many Reflectors are just places where people hang out and test CU-SeeMe, while others only allow receivers to connect, as their sole purpose is to transmit a single source of video to the world. A good example of this would be NASA Select TV which carries live pictures of Shuttle launches and other NASA related material. Reflectors work by taking in video traffic generated by CU-SeeMe clients and resending or reflecting each video packet back out to all receivers connected to the Reflector. This can be quite a large workload and is the reason why Unix systems were originally chosen to perform the task.