I tried what @modemlooper suggested, and there is nothing in that list that allows for underscores to be used. WordPress allows underscores to be used, but BuddyPress does not. I deactivated BuddyPress and was able to register with an ‘_’ underscore in my username. I am working on a hack for this as we speak.
WordPress 3.9.1 / BuddyPress 2.0.1 Installed
@derekfoulk active 10 months, 3 weeks ago