Dip, not sure on the radius thing. Had a group of trammel ghost checking out the fel dungeons Tuesday evening. We where in despise. After you get off the bridge turn North and the first spot you can go up (so you can get to the teleporters). We where all kicked from there. That is several tiles from the alter.

Vepl

ppl could just use "stealthers" its very easy to make a char w/ 50 hiding/necro, put em in wraith form, and hide em wherever near the altar =\

best bet is get tracking or one in you're group w/ 80+ tracking
check for monsters (wraith is monsters!), animals (ninja forms!), and players.
if you find and kill one, then exo it on the next cycle, chances are you have atleast an hour or so before they notice with which to get the spawn done.
Never know really tho.
Risk vs reward! =0

I don't know the exact radius, and even where you were located is still 1 screen away from the alter and in the midst of the spawn. It's moreso a ghost on the outer-area of the spawn is safe and can see whether or not it's been worked up past tier 1 that let's them know.

And Cutter brought up the point of stealthers. (I didn't mention 'cause I do it and didn't want to spill the beans )