Inviting a user to the Portal is done by selecting (highlighting the client or a group of clients) and clicking on the Invite button. This will create an account where the user will have access to the client(s).
See Accounts for further information.
You can also Assign existing accounts to access this client's information, by selecting the client or clients and click on the Assign button. This will present a list of existing accounts from which to select the appropriate person.