In order to send a Group Chat Invite to another account, you must be an Account Owner or Admin.
Open the Relay app
Tap on Groups
Tap on the Group Chat you wish to manage
Tap on Invite accounts to this chat
At this point, you can share the link through multiple apps on your phone to another Relay user for them to join your Group Chat
Adding an account to your chat does not give them access to your account information or setup.
You cannot remove people on the other account from the Group Chat; only their Account Owner or Admin can do that.
You can add or remove people on your own Relay account into the Group Chat; the other account cannot.