1) Determine regular users and assign them Named cal. Uncheck Allow dynamic cals.
Next time if any user wan't to have access to named cal you need to add here manually.
2) Allocate document cal to a document and check allow dynamic cal, So whenever a users comes in he will be allocated a document cal. You can remove document cal if not use more then 24 hours, by this you will be generating space for another user. If at all any user come he will be allocated Document cal automatically if available.