Will Planner ever work for people with On Prem Exchange?

Kelemvor333: I'm not sure if this will give you exactly what you are looking for - but I did find a way to have a "Plan" with other members, even though we still use on-perm Exchange. Basically, I created a Team in "Teams", and then added a tab in it and picked "Planner" from the icons. With that "Plan", I was able to assign tasks in that plan to any members of the already created "Team" - which doesn't seem to have any issues with our emails being hosted on-prem. We are running Azure AD Connect to synchronize all our local accounts out to Office 365, so it does have all the account info including AD groups, etc. So basically, I had to encapsulate the Plan in a Team to get around those errors where I couldn't add members, etc. Also, that plan does show up in my Office 365 Planner page, and then I was able to copy each task in a Plan I had already created over to the new teams-based plan. (Although it won't copy attachments or labels when crossing over for some reason - those options are grayed-out, so I had to manually move those parts.)

Hope that is helpful!