Click on the Members tab
Click Invite a New Member
Put their Email Address, click the checkbox that says Invite as Member then hit Create Invitation
Note: They will receive an email saying that you've invited them to be a part of your organization and they will be asked to create a student only account (if they don't have one yet) to view your mastermind.
Next, go back to the Members tab and click their name
Make sure that the only thing that is checked is the one that says Grant Privileges of Member, choose the title of the mastermind you want to give them free access and click Update Membership.