I don't know much about iPhone, hence the Android forums, but with android you usually have to select to send messages as a group or as individual threads. If individual thread is chosen then each member of the group will get a message as though it was only sent to them, and any replies only go the the sender. If group is left on then each message goes to the members as if sent only to them but replies are sent to the group. Usually that is the complaint, that people are getting messages or replies from people they haven't messaged or don't know, due to the message having everyone's contact info attached to the message. You could ask the sender to check this setting, if on android.