Users can be invited Admins only - to invite a new user go to Settings>Users and hit New User:
Click the Adminstrator to Open Drop Down, THEN Click on the Admistrator Icon to access Users and Groups
Next, you'll fill in their name and email. They'll get an invite via email and be required to set up a Redash account.
Users can be added to existing groups by first selecting the Groups Tab and then selecting the blue hyperlink for the specific group you are looking for.