To invite an external user to a federated DM or channel, use the following syntax to add them as members when creating the room:
@username:server.com@username: The username from the external server(:): A colon character separating the external username and the external server name.server.com.com: The external server name or domain name
For example, @test_user:matrix.org.
When you successfully invite an external user to a federated room, a local copy of that user is generated within your Rocket.Chat instance. There's no need to use the external user syntax (@username:server.com) to invite them to other rooms in your workspace. Instead, you can simply invite the local user, who appears as a suggestion in the "Add Members" input. Once the external user exists and the external server runs, the user will auto-join the room. However, they can choose to decline or accept the invite.
Federated users can’t be deleted on workspaces from 6.12.