Puppy normally boots straight into the root user account. This is equivalent of administrator in Windows. Other user accounts that are available, such as Spot or Fido, are not full implementations of a multi-user Linux.

Pizzasgood made a multi-user version of Puppy 4.21, more information and download available in the forum thread

Puppy421Multiuser
spot
Security

Multiuser Puppy 4.2.1 thread
How to build a Multi-user Puppy
Introducing 'fido' - Barry's blog
Fixing 'fido' thread
'Fido' and 'Spot' for Beginners
Multi user support in Puppy - Comment by NathanF