Put them together somewhere that us neutral territory such as a run on grass (move to a different bit of grass if you've had some recently in it where it currently is) or on the floor in a different room of your house from usual.
Completely clean out, disinfect and rearrange toys etc in the cage they are all going into after meeting.
For a few days at least remove any hideys, cosies etc that don't have more than one way in and out.