Can we dispense with the vaere?
If you are going somewhere, to a party, a store, etc. you can, if you are playing a game for example you cannot.
hvem kunne tænke sig at være med? - should be accepted as well, due to the fact that the person asks who wants to join. The answer could be in this case be "jeg kunne godt tænke mig det"
Hvad om "Hvem vil slute sig"?
"Hvem vil slutte sig til" should be correct as well
Hæren er fint