It should only be an issue if they are in the exact same SO you are trying to print.  If they are entering new orders and you are invoicing existing orders, it should let you.
 
If you go into SO-A and scroll to the top of the list of SO, is there a blank at the top of the list?  Delete it if there is, that could possibly be causing a problem.

??? ???We have been complaining about this problem for months now to no avail.  We have an unlimited user license but only 1 person can enter sales orders and print acknowledgements and packing slips at the same time.  And if they are printing acknowledgements and packing slips, no one else can print invoices.
 
We also cannot release sales orders at the same time someone else is entering.
 

We have been having the same thing happening here- we now have 4-5 people entering and accessing sales orders- whenever someone is in any SO we cannot print invoices.  Once we get the message about file being locked even if we continue to say yes until the file is free it won't print the invoice- it just kicks up a blank invoice.  This is really becoming a problem as we are not getting our invoices printed in a timely manner.