Record Lock When Appending?

It should be noted that this is a shared database with the BE on a network. I have a process that allows users to create an Agenda for a meeting. In order to make the process more efficient the user selects a drop down box which then prefills attendees for that meeting. All very simple really, until more than one user attempts to run the process.

The issue is that this process runs a couple of queries. One to filter the attendees and then another to combine this information with the agenda information which is then finally appended to the MASTER Data tbl.

Is there a way that I can stop users using this at the same time by either giving an alert to the new users saying please wait and defer the process until the current process has completed and then the new request starts.