Gotta lead by example on this one. The 5 man group plays every day, didn't like the PC Discord vs Console Party Chat/ Forum mentality and stick strictly to a group message or specifically targeted discords for all the needs.
We're not getting those guys to post on the forums, I know it for a fact and I won't push them to do so.
Heck, even our regulars here have the same top 4 posters monthly, weekly, etc.
A shout box would be neat, but to be fair I doubt I personally would check it. Discord will always be instant because it's the primary VOIP of most PC gamers.