If you're having trouble sending group text (SMS) messages, you may need to update your account and messaging app settings.
When you send a text message to multiple recipients, most smartphones will send it as one message rather than several individual messages. In order to send it as one group message though, the message has to be converted to a multimedia or MMS message, even though there is no picture or video attachment. Some phones make this very obvious by actually telling you it's converting the message to MMS as soon as it detects that there are multiple recipients.
When you send group messages as MMS, it keeps the replies in one thread on your phone. It also doesn't count towards your text messaging usage, so you won't see those messages reflected in the text messaging detail on your monthly bill.
Since the group message is sent as an MMS message, it incurs data usage and will show up on your monthly bill like the image below: