Group Messaging - iphone 12 mini

I have a group of 4 ladies that I send group texts to. When I send it to all of them at the same time, their replies come back as individual texts, not in my group text and they say that they do not see the text as a group text on their end. I've restarted my phone and that still does not help. Also how can I put these 4 ladies in a group in my contacts? I've tried to figure it out but can't so I end up adding each of the 4 to every group message.