Hi Jim ~
Unfortunately there isn't a way to integrate a membership directly into your SBI site. However, you can set up a separate membership site and link to it from your SBI site.
I sell online courses and needed a secure place to host them that also processes payment, etc. After trying multiple options over the years (EasyMemberPro and Clickfunnels, among several others) I have finally settled on MemberMouse, and it is AMAZING. I can not recommend it enough (and no, I'm not an affiliate or in any way associated with them). There is definitely a learning curve, but the documentation, help forums, and support are fantastic (very much like SBI)...and the entire platform is just incredible. It's insanely powerful and can do pretty much anything you want, once you learn how to navigate around. And, starting at $20 per month, it's also one of the most affordable.
The obvious drawback is that you have to host it from a WordPress page (which, of course, is additional hosting, additional costs, additional learning). So I have my sales page on SBI, then a "click here to buy" button, which takes the customer to the checkout page on my WordPress site, which is where I host my MemberMouse membership site.
I don't know if I'm allowed to post links (they're not affiliate), but it might help to see it in action.
Here's my SBI sales page: [Domain Private]
And, if you click on the "buy now" buttons, it takes you to this page (my WordPress site where I host my MemberMouse membership): http://www.myeventplanningcourses.com/checkout/?rid=p2Di02
Once the customer purchase the course, they are taken into the secure member's area (which is the MemberMouse membership software hosted on my WordPress site).
I am fully aware that I did not make the transition between SBI and WordPress look entirely seamless (mainly because I keep changing the L&F of my sites), and I have heard that InfinIt can help with that; but to be honest it's never affected anything and not one customer has ever said anything.
And again, I have tried many other membership sites (quite a few on trial periods, and I used EMP and Clickfunnels for awhile - I do not recommend either of those) and I can not say enough good things about MemberMouse.
Anyway, it's a bit of work, but this is the most viable option I've found. Good luck!