Blocking DMs wouldn't solve the core issue and is counterproductive. You still want them for coordinating things like one-on-one meetings, for starters. Instea

Blocking DMs wouldn't solve the core issue and is counterproductive. You still want them for coordinating things like one-on-one meetings, for starters. Instead, tie concrete problems for the individuals who prefer DM to a lack of visibility. Hold them accountable for future problems that occur. Oh, that release went poorly? Who needed to be in the loop who wasn't? What can you do differently next time? Yes, getting them all in a group chat would be an improvement, but if you really want it to go smoothly next time, make it visible and public so everyone who has something to contribute can and you personally don't have to run around making sure everyone is in the loop. It's easier for you and it's a better practice. The next time there's a problem that can be attributed to lack of public communication, reference the first conversation and ask why they didn't make it public. Then ask what steps they can take after this conversation to make sure they do it next time. After this it becomes a standard coaching/performance issue.