How is a group selected in a multi-signature?

Question

blockchain

There are different signatures in which several users participate and are collected in one group, multi-signature, ring, group. How is this group chosen to create signatures? Is it just a random set of hosts that randomly gathered into a group? And how long does such a group for signing transactions exist? I didn’t see information about this in the video and books, it was just about recruiting people, but how it is chosen - no