The guest sees "This link has expired" — its time is up. Open the Bridge from "My Bridges" → Open again.
"This link was for one person and is already in use" — the Bridge is for one person and someone joined first. Create a Bridge for several people.
"You have 20 open Bridges" — close an old Bridge to create a new one.
"You can't create Bridges until…" — 3 of your Bridges were closed for violations. Check Tellu's official messages, and contact support if you think it's a mistake.
The guest doesn't see my replies — make sure the Bridge is still open and you haven't blocked the guest. Browser notifications only work if the guest allowed them.