Membership & Multisite- How to add new members

Hi! I have set-up a multisite network. I would like the main site to be the hub for all my sub-directory sites. If someone registers for a new Membership on my site I'll then set them up a site per Membership level/subscription that they choose. I realize I will have to go in and set-up a new site every time someone signs up and that is fine. But I can't figure out how to add new members to the sub-directory sites. Please help! I have been researching for 3 days and found nothing about how to add new members to the sub sites. Thanks!