He doesn't want his supporters to attend the meeting because he doesn't want to be associated with signs like:

"Process Over People"

"Stop Counting the Votes"

"Punish Michigan"

"Florida Should Never Vote Again"

"Honor Rule 20.C.1.a"

Obama's position on this matter is a total embarrassment both for him and the party.  It's pretty obvious why he doesn't want publicity associated with his stealth stance against millions of voters and two major states.

If I were attending the rally, I'd just bring a sign with Michigan and Florida colored in red.