X windows access problem

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How can I get x windows to run under any user other
than root or su? When I logon as a normal user and try
to run my x windows (using startx) the xserver starts
and then it exits because “No valid modes found”. 
When I try to run xdm, it says, “only root wants to
run xdm”; but when I su from this same account
everything seems to run fine. Someone suggested
modifying the “xwrappers.config” file, but I
discovered I don’t have one. Does anyone know how to
fix my problem?

Also I‘m having trouble understanding the user/group
system under Linux. I don’t know how to add users to
groups and how to modify groups’ and users’ access
privileges for certain application (like x windows). I
cant find any useful information in the HOWTO’s. Can
anyone tell me where to look (or just explain it to
me) ? 


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