With BuddyPress Rate Forum Posts plugin your BuddyPress site memebrs can rate forum posts. It highlights good posts whereas poor posts are diminished. Highlighted karma points are visible for each site member. Note: The plugin is only for group forums, it is yet not compatible with BuddyPress site wide forums feature.
Whenever someone registers on your BuddyPress site you can automatically send a welcome message, friend request, group invitation and also you can redirect to a custom page using Welcome Pack plugin for BuddyPress. You can do a lot more, go through this tutorial.