I've spent my career building these platforms. It sounds like you might be better off building your site on top of an existing framework that supports membership: that way you can continue to focus on your startup's differentiating value, rather than rebuilding the wheel with a membership system.

Several are available. Elgg (I was a cofounder) is now a programming framework designed to make it easy to build social sites. There are other good choices - it depends on the features you need on top of the membership functionality.

Let me know if I can help.