If you are running the file as Pro, then no you cannot move to multi-user mode.

If you created the file as Premium or Quantum, then yes, on the File menu there is the option to Switch to multi-user mode.

If you have not yet set up users or a password for sysadmin, just hit Cancel when prompted to add users and you should still switch to multi-user mode.  I haven't tried it on a file in the last several versions but it used to work that way.

Hi AngelaPLKA, 

In addition, to Richard's comments. Please see this resource with the note below:

"...if you are a user of the Pro edition of Sage 50, Sage must be closed down in order to be able to publish invoices from AutoEntry. This is due to the limitations within Sage 50 for the Pro edition whereby only one user can access data simultaneously." 

Hope this helps. 

Erzsi