Go in to the VB Editor, and from the Tools drop-down menu select "Digital Signature", and choose your signature and then save.
The first time someone opens this file, they will still need to accept your digital signature. If they select "View Certificate" and go through the whole process, and at the very end select "Accept all documents signed with this signature", they will not need to do it every time. Every file signed by you will automatically be accepted and macros will automatically be enabled for all files signed by you (for all files they open on that workstation).