There seems to be some confusion about this song, at least for fans born after 1990 or for those who aren't as proactive on the web as they could be and wonder why it wasn't on their copy of Marvin's.
"Just A Friend" is an old, quirky rap song done by a guy named Biz Markie.  This was his biggest hit in the 80's.  I'm sure you'll have no problem finding a way to listen to the original version.
A live performance of the song was included on The Pingry EP. As for the studio recording,
"A studio recording of "Just A Friend" was made in July '06 but did not surface until the Atlantic Records releasing of MMMM in April 2008.  Those who attended concerts after the release of the album were given instructions on how to download the track for free."
Basically, you were given a small business card with a URL and password to download the song for free.
Later, in August 2019, after years of inactivity, the band came together to make the combined decision to distribute  Just a Friend  on all popular streaming platforms for the first time.