When logged in as the admin go to create a Member.
Then, on the right side enter the email address of the Member you want to give the free account to.
It will send them an email inviting them to signup for free.
Under advanced features
change "Software Is Free" to No.
Then, on the join page (/join), you can either enter the email addresses of the people you want to give free accounts to on the right form
OR fill out the left form to create a free account.
Also, you can import several members at once from .csv file using the import members section.