I train them together and separately. If I call the whole group it's "Doggies, come!"
Usually they all come when I call one dog but only that dog gets a reward. Training them together teaches impulse control: one in a down stay while working / playing with the others. Also let them out of the door /car by calling their names individually. I love it.
Deja, GSD, sable WL, female, born 11 / 2013
Cam, Rough Collie, sable, male, born 8/2015