BuddyPress.org

There’s a “Sign Up” link notice in adminbar if you allow registration, beside the “Login” link see http://testbp.org/

Registration is disabled by default, so if you’re not seeing the Sign Up link, you should go to admin dashboard > Super Admin > Options – Registration Settings – Allow New Registrations – check any other than default “Registration is disabled”

Edit- for single WP install, go to dashboard > Settings > General Settings – Membership – Anyone can register and below that, you can choose Suscriber role.