ok, basically the same thing said before except works for one unit instead of a group, but this might work for you...
place a game logic. Remember that the game logic is the center of your circle, so place it in the center of the arena.
copy this script into a file:
;_gl is the name of the game logic that should be the center point
_gl = _this select 0
;_rng is the radius of the circle, probably about 200
_rng = _this select 1
;_u is the name of the unit to teleport
_u = _this select 2
_p = getpos _gl
_p set[0, (_p select 0)+(random _rng)-(random _rng)]
_p set[1, (_p select 1)+(random _rng)-(random _rng)]
_u setpos _p
this should work, just place the script into a script file name "teleport.sqs"
then put something like this in a trigger:
[gamelogic1, 200, player] exec "teleport.sqs"
make sure that the trigger can be activated in some way
Hope this works,
Bluelikeu