You only told us about those two systems... what other computers are on the homegroup?
If windows 7 controls the homegroup then 8 can't see it and if 8 controls it then 7 can't... the two systems don't snyc. The work around I posted is to allow a folder to be usable from both 7 and 8... I show the default 'everyone' setup but you could adjust that to whatever group or computer needs to be included or disallowed.
p.s. the point of a homegroup is to make file sharing workable... a server is best if you want to control whom can or can't see those files but that also adds lag to the network.