anonyMous was a comedy troupe that Joe was a part of (possibly was the founder of) while attending U of M.  Rob was also a member, as well as a couple other friendly faces we see scattered around Tally Hall's videos.
In fact, many of Tally Hall's earlier videos were actually taken from a movie called Sargasm, which if I remember correctly, was a compilation of many different anonyMous sketches... not unlike Tally Hall's Internet Show.
Most people become familiar with anonyMous after watching the music video for Break It Down, which is often credited to Tally Hall, even though it technically is not a Tally Hall song.*
*In 2006 Tally Hall did rewrite this song, adding in a chorus and basically made it into a standard pop song. They played it a couple times, but it quickly disappeared from their set.   No word on if it will reappear but it does seem to be a favorite amongst Tally Hall fans.