You can add mentors to the project by adding their name during project enrollment. For details, see the Project Mentors row of Program Setup section of Mentorship Project Enrollment form.
You may receive an email informing you that a mentor is interested in volunteering for your mentorship program. Review mentor's profile and if the mentor is a good match, click on Add to Project CTA in the email to navigate to Edit Project form where you can add the mentor to Project Mentors section.