[dba-SQLServer] Users in SQL Server

Several times now I have run into an issue where SQL Server does not have a pair of users I use for 
my Access application called DiscoAdmin and DiscoApp.  The problem is that one of the the databases 
that I am picking up on that server was already set up and has those users.  When I try to set those 
users up in the server's security it tells me that "the server principal 'DiscoApp' already exists. 
  Basically I am able to create the user but not assign rights  - db_reader, db_writer etc.

I have always been fuzzy about how this stuff works and I just need an explanation of why it won't 
allow me to set these rights even though the user exists and the check boxes are enabled when I 
select that database.  I just discovered that if I delete the user out in that database and then set 
the rights through the user back in the server security stuff it happily accepts my changes and 
re-adds the user to the database.  This just seems strange.  If I don't do this rigamarole then I 
have a DiscoApp in the database with a set of rights for that database but when I look at it back at 
the server level it does not reflect those rights for that user for that database.

Any assistance great fully accepted.
-- 
John W. Colby
Colby Consulting

Reality is what refuses to go away
when you do not believe in it