This is a management room: prefixing commands with !tg is not required.

General

cancel - Cancel an ongoing action (such as login)
help - Show this help message.

Authentication

username <new username> - Change your Telegram username.
displayname <new displayname> - Change your Telegram displayname.
session <list|terminate> [hash] - View or delete other Telegram sessions.
ping - Check if you're logged into Telegram.
ping-bot - Get the info of the message relay Telegram bot.
register <phone> <full name> - Register to Telegram
login - Get instructions on how to log in.
logout - Log out from Telegram.

Creating portals

bridge [id] - Bridge the current Matrix room to the Telegram chat with the given ID. The ID must be the prefixed version that you get with the /id command of the Telegram-side bot.
create [type] - Create a Telegram chat of the given type for the current Matrix room. The type is either group, supergroup or channel (defaults to group).
pm <identifier> - Open a private chat with the given Telegram user. The identifier is either the internal user ID, the username or the phone number. N.B. The phone numbers you start chats with must already be in your contacts.
join <link> - Join a chat with an invite link.

Portal management

config <help|subcommand> [...] - View or change per-portal settings.
invite-link - Get a Telegram invite link to the current chat.
upgrade - Upgrade a normal Telegram group to a supergroup.
group-name <name|-> - Change the username of a supergroup/channel. To disable, use a dash (-) as the name.
delete-portal - Remove all users from the current portal room and forget the portal. Only works for group chats; to delete a private chat portal, simply leave the room.
unbridge - Remove puppets from the current portal room and forget the portal.

Miscellaneous

sync-state - Fetch Matrix room state to ensure the bridge has up-to-date info.
id - Get the ID of the Telegram chat where this room is bridged.
search [-r|--remote] <query> - Search your contacts or the Telegram servers for users.
sync [chats|contacts|me] - Synchronize your chat portals, contacts and/or own info.
play <play ID> - Play a Telegram game.
vote <poll ID> <choice number> - Vote in a Telegram poll.

That Telegram chat already has a portal at #telegram_appliedcategorytheory:t2bot.io. Additionally, you do not have the permissions to unbridge that room.

In reply to @davidad:matrix.org
!tg login

login is a restricted command: you may only run it in management rooms.

In reply to @nickhu:matrix.org
erm, you probably want to do that in a private chat with Telegram Bridge...