Do you have an iPhone (or other iOS device or a Mac computer)?
Are you signed in with your Apple ID?
Do you have iMessage on that device? On an iOS device, have you gone to Settings > Messages and turned on iMessage?
Under "Send and Receive", do you have your phone number (or email address) selected?
Are you in a country where iMessage is prohibited?
Has someone else in the "group" added you to the conversation?