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.

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.