I don't send group emails often, so I just ran into this problem recently. It appears that after the upgrade to Froyo I am unable to add all of the contacts in a group into an email (using Android's stock email program via POP). I have a distribution of 36 people that I was able to send an email to by simply selecting a group in the past. Since Froyo, only 8 of those contacts are added to an email when selecting the group.