How do I allow users to register to my site?

This is a general WordPress option that can be found under Settings -> General -> Anyone can register.

If you're using BuddyPress the register page should have been created automatically. If it has not, create a new page called "Register" and publish it. Now go to Settings -> BuddyPress -> Pages and select this page from the Registration dropdown menu. Now you can link to this page in your menus from Appearance > Menus.