What is the best method for reading/writing to an excel file using VBA where
multiple people need to write while other people are reading.

1) Will GetObject work or do you have to use some other method?
2) Does the file need to be locked to prevent multiple users from
simultaneously writing to the file?
2) Most users know how to use Excel and probably would prefer Excel, but
is there any advantages of using Access?

The writes to the file will just add data to new rows at the bottom of the
worksheet. The writes will be done with VBA, and the Reads are just people
looking at the data and no VBA will be required.