Cannot open X display from other userid

Using Redhat 6.0 on a Pentium, when I login
as me (not root) and startx, then start an xterm
and in the xterm I issue:
su - othername

then othername works fine, EXCEPT it cannot open the
X display.  I get messages like:

Fatal Error in gearspgm: could not open display:

or from Emacs:
Error in init file: error: "X windows are not in use or not initialized"

% xclock
Error: Can't open display:

% echo $DISPLAY
localhost:0.0

I tried issuing, from my userid xterm:
$ xhost +
access control disabled, clients can connect from any host

But that didn't help.

How can I get X working for other userids as well?
Many thanks -

Steve