You can find the Users section in the Admin panel of the Back Office. There, you can manage users, their relations, Roles and Policies.
To invite users, go to Admin -> Users and click Invite members in the top right corner.
To send invitations, fill out email addresses one by one, or use drag and drop to upload a file with an email list, then click Send.
Invited users will receive an email message with a registration link. With it, they can register and create their account in the frontend as customers or in the Back Office as members of the team.