Macro for creating data validation input messages

I'm new to VBA coding and have been pretty much getting help/researching putting together this interactive calendar (which sounded easy but has turned out to be very difficult).

One of the last steps to this project is that I need to show an audience impacted over the cells the project takes up on a calendar.

The best way I've seen to do is it either via comment or data validation input message and I'm going to go with data validation input message as the comment indicator color makes the calendar not appealing.

I've attached images of the calendar and the spreadsheet its pulling from.

The range in the "High Impact" category of the calendar is named "HighImpact" and if I wanted to, for example, look at the audience of "Project A - Kevin" from D4:O4 and when they click the cell, a data validation input message would show the values in the spreadsheet "HI Project Database" and the values from cells AB2:AG2 with commas in between would appear in the input message.

Any ideas on where I can get started or a reference thread?