In order to send a Group Chat Invite to another account, you must be the account Owner or an Admin on the account.
Open the Relay app
Under the Group Chat list, tap on the chat you wish to manage
Tap on the Group Chat Icon at the top of the screen
Tap on the magenta icon to the right of Invite accounts to this chat
(At this point, you can share the link through multiple apps 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 specific people from a Group chat. In order to remove a person from the Group Chat, you can either delete the Group chat, or they will need to remove themselves by disabling the chat in their settings.