Why can't I join a group chat with people with iphones?

I have an android phone(moto G) and when one of my friends tries to add me to a group chat of all other people with iPhones it won't let me join. Why can't I? How can I fix the problem?

And yes… I do have group messaging turned on.

Iphones have imessage which has special built in features (which are really good features also…). Other phones don't have that.

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