You can do this one of two ways:
Through Point of Sale > click Home > Point of Sale > choose the customer > add the subscription or pack to the cart > under payment method, choose free buy.
From the subscription/pack page
Click Memberships > Subscriptions/packs > select the membership > scroll to the Active signups list > click New sale > search for the customer > choose free option.
Note: for both options listed above, if the subscription is a renewing subscription, the customer will continue to receive the benefits free of charge every billing cycle.
If you would like to set an expiration date to these benefits or a planned price increase, head to their profile, click the dropdown menu to the right of the subscription and click Edit.