Coda has its own users and groups. These are internal to Coda; they are not the same as Unix users or groups. Coda users correspond to positive integers, while groups correspond to negative integers. Your life will be easier if most Coda users and Unix users have the same names and ID numbers.
The program `pdbtool' manages users and groups. It must be run by root on the System Control Machine.
Coda groups have owners, and both users and groups may own groups. Under AFS and DFS, any user may create a group, delete the groups he owns, and add other users to his groups. None of this works in a Coda cell, and this is in my opinion something that needs to be fixed. At present, a cell with many users will probably be more of a burden to its administrator because of this.