File ▸ Live Share… (BETA) and the notebook's context menu share it: Start
Sharing gives a code like 412-violet-otter, optionally with a password, then
lists who is connected; Stop Sharing lets everyone go and retires the code and
secret. Open Shared Notebook… (File menu and the welcome screen) takes the code
and opens the notebook through the host, kept at live://<share> and opened
from its last listing while the host is away; a share and the notebooks joined
come back after a relaunch. Wrong codes, codes no one shares, expired codes,
too many tries and an unreachable relay each say so.

Presence is on for notebooks others reach too (on a server, in iCloud Drive,
in another computer's folder, by Live Share), in the shown notebook's room
only, and the mark beside the avatars says others see you. Options ▸ Sync &
Storage ▸ Live Share turns it off, leaves the account picture out, or names
another relay. Menu items take a badge (ui::badge); macOS's bundle lists
_snowbound._tcp for Bonjour.

Assisted-by: claude-opus-5.5