Under the menu you can click "Members." On that screen you can click "invite member" and then add the user's first name, last name, email address and role (member, admin, or member consume-only) and click "Create Member". They will receive an email invitation to the organization.
You can also select "Upload Members" which will ask you for a CSV file with multiple users allowing you to bulk upload users. A bulk upload template is attached.
On the Members screen, you also have the ability to edit the user, delete the user, and report on the user's activity.