Group Messaging Missing?

I have an iPhone 5c, I am able to use iMessage fine with other iPhones or not they both work. But I cannot be in group chats, start, or receieve any messages from the chat. my friend said she already added me in a group chat but i dont receieve or get anything.. I check all the settings Mms is enabled no one is blocked and my phone number is for send and receive. On my friends phone they have an Group Messaging button enabled but on my phone there is NO button the says "Group Messaging" like my other friends. How do I fix this and I use ultra.me as a carrier