Having read through the Nest Multiplex API guide here:
https://developers.nest.com/documentation/cloud/multiplex-overview
There are several mentions of something called a "multiplex group connection".
Verbatim, from the doc:
Each multiplex connection should have 10-20 user connections. As you build more multiplex connections, you can gather them into a multiplex group connection, which should be limited to <300 connections.
They show very clearly what a multiplex connection is (basically just invoked with a curl command.) But there is no mention of what a group connection is, or any other details.