su question

[Linda]

I have several timeshare employees that I would like to have share the 
same email account but not the same login. I thought I had a solution 
figured out by moving the thunderbird-profile to a directory they could 
all access. However thunderbird ignores umask and on closing sets the 
inbox as user rw no group permissions.
I thought I would solve my problem with thunderbird resetting the inbox 
permissions different than umask by having my different users access the 
same thuderbird profile using su. I could just create a user for the 
thunderbird account set it up the way I wanted unfortunately it does not 
work.
If I type  su username -c thunderbird     instead of opening thunderbird 
I get this

No protocol specified

(thunderbird-bin:9606): Gtk-WARNING **: cannot open display: :0.0


any ideas on how to get this to work.  These are users that I do not 
want to give any special permissions to.

                              Thanks