Can't Reply to Group MMS on my Galaxy s5.

[Android Central Question]

I receive group mms texts where I see many people replying, but my send button is greyed out. And I see a message that there is a 10 person limit. I can text and reply to other group texts..... but not from this sender. I can text with him when there is no group. Not sure if it is because my phone is old, or they all have iPhones and I have an Android. The group messaging is turned on btw. Ideas?