You can enroll customers directly into your product just by entering their email address. They'll receive an email that they're now enrolled. They'll then click a link in the email to set a password and create their account.
Step 1: Go to Products
Step 2: Enroll the customer
Click on the three dots to pull up a menu with various options:
Click on "Enroll customer".
A window will pop up asking for their email address:
Type in their email address and click "Enroll customer" to give this person free access to your product.
Your customer will get an email that looks like this with your logo and color palette:
All they'll have to do is click the link to set a password. If they already have an account with you, they'll receive a welcome email instead with a link to login.
Note: This will override any start date or email campaign you have set for the product.