By default, K2 is set to use Active Directory as its user manager ("ADUM"), but K2 also has a separate SQL User Manager (SQLUM) component that can be used.
Using an alternative user manager ('UM') other than Active Directory can be done with K2.net 2003, but requires K2 to know where that repository lies.
Currently, K2 only supports one or the other mode (ADUM or SQLUM), but can't mix the two concurrently (also known as "Multi-User Manager" or "MUM"), Impleenetation of a MUM fuctionality is provided via custom development, typiccally provided by K2 partners.
Download the K2 SQLUM from the Portal. Once you download and install SQLUM (it changes the User Manager settings on the K2 server), you can then create the users and groups in the database and then set the ASPX forms to "Forms Authentication" and it will go thru the SQLUM database to validate the user login and pass those credentials on to the K2 Server for interacting with the workflows.
Hope this helps.
- Dave
Dave