Currently you can add members manually to:

• Free Memberships
• One Time Payment Memberships
• Trial Memberships

However Admin is not able to manually add a user to Subscription based membership level.

The logic behind this is that the user must enter his/her payment details into the system and will be billed as per the subscription period set by Admin - If payment details are not collected (which they cannot be in the case of manual addition of a user by Admin) then no subscription billing can occur. 

As Instant Member locks out users who cancel or default on subscription payments automatically if an account were manually added in this way the user would be automatically locked out at the first instance of rebill.

If you wish to give people free access to your site then you must create either a 'free', 'one time' or 'trial' level membership to do so.  (As Instant Member supports unlimited levels of membership you can configure multiple levels of these type of accounts.)

Alternatively if you wish to give users free access to your subscription based membership level you can create a subscription style account and offer your user a coupon code set with a 100% discount.  (Create a coupon code under 'Coupon Manager' from inside your Admin panel).

If you wish to offer a reduced monthly fee you can do this also using coupon codes.