iOS 8 group chat sends as text message on iPhone 5s

My iMessage group conversations were working fine before I updated to iOS 8, but since I updated they have not been working. I have been able to receive messages sent by others on the group chat, but if I want to send anything I am forced to send it as a text message instead of an iMessage, meaning that each person in the group chat gets the message individually on a private conversation just between myself and them. I want to be able to send iMessages on group chats like I could before updating to iOS 8. Can anyone offer any solutions please?