You can add a colleague, subcontractor, or external advisor to a location if you have the Manager role or above and your organisation's plan includes the External User Access entitlement.
To do this, go to Users & Teams, use Add team member above the list, and in the Invite User panel turn on External User if the person is outside your organisation. Enter their email address, choose an appropriate role, then under Location Permissions use Add Location to grant them access to one or more locations. You must grant at least one location for external users; otherwise, the invitation cannot be sent.
If you do not have permission to invite users or manage their access, ask a Manager, Admin, or Owner in your organisation to add them through the Users & Teams section, where they can also set roles and location permissions.