Ok. Now that one is doing some weird stuff at the moment.
I changed the stand-in user id of 5 to the ParamUserID in both locations. And added tblbooths.booths_id to the select. But other than that I used it as is. And what I get is all event dates showing up in the drop down at first. But as soon as the user registers for even one date, they all disappear accept one date. And that one date depends on who is logged in.
And if all three users are registered for the same date, all users loose any dates as a choice. So the dropdown is now blank for everyone. Strange.
For reference, this is what I tried from your provided code. (I'm still trying other stuff but wanted to give an update).
SELECT tblbooths.booth_id, users.UserID, show_date, UserFirstName, UserLastName, reserved
left JOIN tblreserved ON tblbooths.booth_id = tblreserved.booth_id
left JOIN users ON users.UserID = tblreserved.UserID
WHERE (coalesce(tblreserved.UserID != ParamUserID,1=1) AND (coalesce(tblreserved.booth_id NOT IN (SELECT booth_id FROM tblreserved WHERE UserID = ParamUserID),users.UserID>0)))