If the people in the group you're sending an SMS/MMS to are using an Apple phone as well, it shouldn't be a problem.
If just 1 person in the group isn't using an apple phone, it won't work with Optus. That said, apps like Whatsapp, Viber, etc would all facilitate this for you and most are very cheap or free.
Hi Sam: Thank you for this. It is challenging to find out if this is the case. I am also not seeing this feature in my Settings>SMS/MMS options anymore. Your feedback is much welcomed. Heico