I'm on the iphone 4s and started experiencing similar problems after updating to 7.03. I can group iMessage to other iphone users with no problem. The problem exist when I include a non-iphone user, ie Android, into the group. Everyone (iphone & andriod) receives my txt but when they reply back, I don't receive any txt at all (including txt from the iphone users). They'll think I'm receiving but just not replying until someone calls me. Everything works as expected when I individually txt people (iMessage & SMS) on either iphone or android platforms.
I believe this problem exist on an individual basis and not on a mass scale - at least not yet.
In one of my research, a friend (iphone 4s on Verizon) sent a group SMS to me (iphone 4s on AT&T) and one other person (android on AT&T). The sender, on Verizon, was able to recieve and reply back to the android user as expected, whereas I did not receive anything, not even the initial SMS message.
This led me to think maybe its an AT&T issue, so I started a new group SMS to an iphone 4 user who's on AT&T and an android user also on AT&T. They both got my initial txt. They were able to send and recieve to the group with no problem. I on the other hand, did not get any txt back from the group after my inital message. It seems like when I'm going through SMS instead of iMessage during a group txt, I can send but can not receive. This really mess up all the group SMS alerts I usually receive during the day.
I have relaunched iMessage and reset my Network settings and still the problem exist. My next option is to reset the phone, which I'm not looking forward to.