BuddyPress.org

https://buddypress.org/community/groups/bp-xtra-signup/ may work

However, another option would be to add a line of code that asserts that if a user proceeds in creating an account, they agree to your Ts and Cs. Find `registration/register.php` and add text to line 198. Surround your Ts and Cs url with a link for users to view it before proceeding. That way any members on your site will have read/agreed to them.
http://www.htmlcodetutorial.com/linking/_A_TARGET_95y98y108y97y110y107y.html