This article https://textra.uservoice.com/knowledgebase/articles/219062-having-issues-with-mms-group-chats does not mention the possibility that MMS does not work properly on your /friends'/ devices. (In which case they won't even see the message, but perhaps a provider SMS telling them that they can view their MMS online.) Unfortunately, this is quite likely, in my experience!

This is the reason that I would prefer that MMS for Group chats was off by default - or actually (even better) if it shows you this option and allows you to choose, the first time you try to send a group message!