To add a new user to your WordPress website, visit the admin dashboard, then select the Users > Add New option from the menu.
Make sure the new user has access to the mailbox associated with the email address as WordPress will send any correspondance including activity notifications and password reset instructions there.
It is also important to select a role for the user appropriate for the level of control you wish to grant them on your site. Authors can publish new posts for example, whereas contributers can merely add draft posts. More information, including the full capabilities of each role, can be found on the WordPress support site.