BuddyPress.org

Make sure you have read this first: https://codex.wordpress.org/Plugin_API

Here you can find a listing of most (all?) of the current WordPress hooks: https://codex.wordpress.org/Plugin_API/Action_Reference

Here you can find a listing of BuddyPress hooks, although this is out of date:

https://codex.buddypress.org/developer-docs/action-reference/

As far as your particular project, you probably can find the proper hook to tie into by searching the register.php file in the BP default theme. There are many hooks you can use.

Also, if you want to learn more about the current BuddyPress hooks, I would recommend searching for all “do_action” and “apply_filter” calls within BuddyPress.