GetObject if you only need to read the file.
Something like this:
Dim wb As Workbook
Set wb = GetObject(strFilename)
'Change this line to reflect the ranges you need to copy/paste.
Using this should allow you to copy from the workbook whether
it's open by another user or not (including you).
I have noticed that this approach doesn't work if the
workbook is protected or if the sheet you're trying to copy from is
Also, only use
ThisWorkbook like I did above if the code
will be in the same workbook as the sheet you want to paste to.