As an administrator of an account, you can add someone to an account by clicking the “Add New” button at the top right of the people section. Create a new person with a role of User or Administrator, or upgrade an existing person with a Contact role to a User or higher.
The person will receive an email notifying him of the invitation and allowing him to set up a password (see Confirming your profile).
Until an account is confirmed, "Invited" will be displayed next to the name of the person as an indication that an invitation was emailed.
To resend an invitation to an unconfirmed user, click on the person in the people list and press "Resend Invite" at the top right of his detail page.
A notification will appear at the top of the page indicating that the invitation has been resent successfully.