Are you using the AT&T message app or the Samsung Message app? I don't think AT&T message app can do group messaging. Samsung one on AT&T cannot either unless you have the newest 4.3 Android OS updated. I have read that 4.3 Message app now allowed group messaging. If you don't have it, you will have to download a 3rd party app that support group chat like Handcents.

As for some iPhone cannot send you txt... did you turn OFF your iMessage setting on your Apple account? You will also need to log in to you Apple account to remove your phone number from it. And on top of that.. some of your iPhone friends may have your apple ID in their contact instead of your phone number. So if they try to text you... it will try to go to your iMessage. Apple made it really really hard to switch away from an iDevice.

 

Try Handcents app. That's what I use. I also have the iOS 7 skin and plugin so my message app actually looks like my wife's iPhone 5s Message app.