Merging Shared Worksheets

I have a master timesheet that i would like to update for several employees. Each employee will be sending their own spreadsheet at the end of the month. the columns in the spreadsheets have the date, the employee's name, the project they worked on, and how many hours they worked on that project.

when i receive all of the employee's spreadsheets (they will be emailing them to me) then i would like to import the data into a single table that i can then evaluate using pivot tables and pivot charts.

I tried using sharing and merging but i can't figure out how to have their information merge into a single table without overwriting each other (the numbers of rows with information is different for each employee and is not predictable - it will vary from month to month).

I would like to make this as automatic as possible. Ideally, they will send their spreadsheets to me and i would like to have their information automatically imported into one big table.