Creating a group on Session requires a server with a clearweb IP.  It’s essentially just a website.  Go to this link and you can literally read the conversation from our group chat:
https://session.simplifiedprivacy.is/r/simplified/

All Session is doing is for group chats is piping that website into your client.  
On the other hand, the one-on-one chats go to a nodes.  And so system we’re presenting here is completely separated from a physical location and the ability to take it down.