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.

You need to be able to get feedback on your product as quickly as possible, so my advice is to choose the technology stack that will allow you to build a prototype efficiently. There's no right answer here: for some people it's LAMP, for others it's node, for others it's a Windows stack. Worrying about the technology at the stage you're at is a red herring: worry about the product, the problem it solves and the user experience of your solution, and get feedback you can iterate on as quickly as possible. I'm a serial startup CTO who's now a startup founder and CEO. Let me know if I can help.