I have tested it just now:

Started the following group with dungeon tool: 1 tank - 3 dps (hunters)- 1 healer:

Dropped the tank and healer (loggod off and then uninvite) and logged in with the 2 other hunters and invited them to the instance group. The 2 new hunters now can enter the instance with the dungeon finder icon. (minimap) Now 5 hunters are in the instance and I have just completed ragefire chasm.

But only the 3 original hunters received the dungeon finder reward.

So, with a little hassle..... it can be done.

Thanks for your help guys.

Regards

MC