Let's start with the basics: My platform - Galaxy S9+ using Verizon's Messages app (Version 8.3.3).
You've received a Group MMS message and you want to remove (or add) one or more recipients. First, open the group text (as if you were going to reply). Then tap on the Recipients bar at the top of the thread. This will pull up a screen with a couple of options followed by a list of Participants. On my platform, at least, swiping left or pressing/long pressing on a recipient does nothing (actually, pressing allows you to view the contact's info). Third, you'll have to select the option "Create message". Sorry folks, this is the only way I've found to easily edit the list. Once you've created a message (sorry, you'll lose the thread history as well) the recipients field will pre-populate with all of the original recipients. The cursor is now flashing at the end of the list so you could now easily ADD a recipient at this point. Fourth, to remove a recipient, scroll left or right to the one you want and long press it. An options box should appear where you can "Remove Recipient". Contrary to what Verizon says, this does not immediately remove the recipient (not on my platform anyway). It simply moves it to the last (far right) position in the field where you can NOW simply backspace the cursor (remember him? He's still flashing there!) to delete the offender, LOL. Repeat for any others you wish delete or start typing to add and voila, you have a modified Group MMS message recipient list. Hope this helps. My searches on this topic came up somewhat lacking so I just experimented until I figured it out. Happy messaging!