Well, Access (like most database apps) is constructed to allow multiple users concurrent update. But I don't know of any other Office app that does that. It doesn't matter (AFAIK) that you're writing a VBA program to do it, because VBA is going to invoke Excel to do the actual updates—your VBA program is "calling" Excel—and Excel is going to complain, when you try to update a workbook, that someone else is currently using it. And likewise it doesn't matter that the request to update the workbook is coming from Access; the request still goes to Excel, and Excel still knows better than to let two people update a document at the same time. If you want true concurrent update, it has to happen in Access, to an Access database. And yeah, you can get Access to update the Excel workbook later, for reporting purposes; but only at a time when no other user happens to have it open.

Wait a minute, I just thought of something: You said other people, up to 12, may be using that workbook. But are they updating the workbook, or only reading it?

If they're only reading it, not changing anything, then they can open the workbook as Read-Only. Then Excel can update it. Would that work for you?