On the Users tab for the tenant, you can manage existing users and add new users.
Your new user will receive an e-mail where they must click the Accept Invite button and fill in their name and set a password.
HINT: If a user should no longer have access to the portal, you can remove the user from the tenant. To do so, on the Users tab, click the name of the user, then click Options, then Remove user. If the user you are removing has content, you can move the content to another user.
If you have created a new user and this user has not accepted the invitation e-mail, you can re-send the invitation to the portal.