You can manually enroll members directly into a free Membership Plan just by entering their email address. They'll receive an email that they're now enrolled.
Step 1: Go to Memberships, on the top navigation menu
Step 2: Click on Plans
Step 3: Click on the free Plan you want to enroll a member into and enroll members on the "Enroll a member" box
Can I enroll members into a paid Membership Plan?
While you can create a 100% off coupon and email it to them, you can't manually enroll someone into a paid Membership Plan.
The reason for this is that paid Membership Plans create a subscription in your payment processor, and there's currently no way to manually enroll the member into a paid plan without doing so.
It's possible that in the future you'll be able to, if payment processors allow us to create paid Membership Plans without automatically generating a subscription.