to their level.  That will tell you the highest level person they can
group with.  For example, a level 40 person can group with a level 60
and still get exp, but a level 42 can group with up to 63.  I remember
this formula was relatively accurate when my girlfriend was levelling
up and she couldn't group with me for experience until she hit 43 since
I was 65...I'm not sure if it's different at higher levels now that
they've been added, but I suspect not.  In addition, this formula
definitely doesn't work at low levels, say below 20.

In LDoN adventures, all the participants must be within 7 levels of
each other (i.e. no more than 7 levels difference between the highest
and lowest level person).  I belive that for the DoN tasks this was
upped to 10 levels, but not sure if that is live yet (based on Dev
posts on the main forum).

Game on,

The Oneiromancer

All correct answers believe it or not!
It's a sliding scale based on your level, the higher the level the wider the
gap may be ;)

If you mean eq2, which based on the question, I am guessing is the case,
then I believe 7 is the maximum level difference where you can still get xp
and quest credit.