BuddyPress.org

This isn’t really a BP-related question, but a robots.txt file is usually not necessary; it’s only needed if you want to block certain bots from your site.

More info here:
http://www.google.com/support/webmasters/bin/answer.py?answer=156449

Also make sure you’re not already using a robots.txt plugin like:
https://wordpress.org/extend/plugins/irobotstxt-seo/