That's because it's not possible. You might be confusing the option or how it worked (if it was in the S8). If you send out a true group message, that goes to everyone can see everyone else's number in the group and everyone can reply to all. If you send out individual messages to a group, recipients only see their own number and when replying, it only goes to you (because you didn't send out a group message (chat), you sent out multiple, single messages to a group (bulk)).
Most 3rd party apps, like Textra, do have this option (to send a single group message with multiple addresses in it or to treat groups as 'bulk' and send out an individual message to each recipient in the group instead.
On the S9 app (which should be very similar to the S8) the option is in the Messages app settings, More Settings, Multimedia Messages. There, make sure that the 'Group Conversation' toggle is set to OFF if you want to send out bulk messages to each recipient (replies only to you) or keep it ON to treat groups as groups, sending out one message to all recipients (replies to all).