BuddyPress.org

This thread has an example of how to build a filter: https://buddypress.org/forums/topic.php?id=2146 for the user full name.

https://codex.wordpress.org/Plugin_API for docs on wp actions and filters.

You should really talk to DJPaul here on the forums. He has a plugin called Welcome Pack that assigns new users a default friend and default group. He also says that the next version of that plugin will have the ability to create a custom welcome message to the new user. Not sure if that’s a PM or an email. Suggest what you want to him. He’s working that kinda track.

https://buddypress.org/developers/DJPaul/

https://wordpress.org/extend/plugins/welcome-pack/