There are three ways users of any email domain can be added to an Organization:
Domain-based Auto-Add: Users that are part of an email domain that’s owned by an Organization will be automatically added when they first sign in. If you’d like to turn this off, get in touch with our team at email@example.com, and we’ll set it up for you. Note: Auto-add can be enabled only for one Organization per email domain.
Admin Console: On the Users tab, an Organization Admin or Team Admin can add users, simply by clicking "Add User" then entering users' email addresses.
External Directory Integration (pointed at the Organization): Currently, there can only be one Active Directory (AD) Sync to an Organization. Please contact your Customer Success Manager with questions about syncing to your external directory.