This requires a 3 step zap.
1. First trigger: The trigger of the Zap is probably the payment confirmation and it contains the email, name and preferably the username of the you customer.
2. Create User: The first action is to create the user. In this case you will be defining a password and the user is required to reset the password to be able to access the community.
3. Add Group Member: Since the ID of the created user is available on the second step, you can just add a custom value for User ID and use the information on the second step to add the user to the specific group.