If you want to allow user bob to access your existing X session from
the command line, you can enter these commands at the terminal prompt:

xauth nextract - $DISPLAY | sudo -u bob xauth nmerge -

Thius uses the xauth command to
extract the authentication information for the current user (first
part of the command, before the |) and then merge it into user
bob's $HOME/.Xauthority file. Note that it requires
/etc/sudoers to be configured so that you are allowed to run
commands as user bob (this is the default if you're the admin user
on a Ubuntu box).

You can then switch to user bob with su -l.

Alternatively, you can issue these separate commands:

xauth nextract - $DISPLAY > /tmp/xauth.temporary.file

su -l bob -c 'xauth nmerge - < /tmp/xauth.temporary.file'

rm /tmp/xauth.temporary.file

Note that:

1) the temporary file must have permissions such that user bob can
read it (which often means that any user on the system can read
it);

2) there is a time window during which any user that can read the
temporary file can get access to your X display.

Therefore, it is better to use the sudo approach as it does not have
these downsides.