I couldn’t tell from the screenshot, but does Slack wait until after you’ve posted to tell you about the problem? I like their UI, but better would be to know before you post, because you might not want or be able to add the user to the group, in which case the post might not make any sense and you’d PM instead. It’d be annoying to find that out only after you write and post the whole thing.
Can the user be greyed out with some explanatory text? Without the explanation, I suspect people will not figure out why they can’t click the user, and just think something’s broken.