Have them open Access, then go to File Open.  In the dialog select the mdb,
and then in the lower right click the dropdown beside Open, and choose Open
Exclusive.

Are they trying to modify the design of any objects?

There are several possibilities.  1.  Whoever created the
database set up record locks for the forms and objects.  
If you plan on having multiple users you will need to
change this.  2.Sometimes Microsoft Access, like
everything Microsoft, is just plain buggy.  Check the .IDB
file (should be in the same folder as the .mdb file with
the same name)  open it with notepade, it should tell you
what machine is causing the problem.
P.S.
MICROSOFT BLOWS