If the herd is all doing the same thing, e.g., playing with the ball, one would say, "The herd of elephants is playing with the ball."
If the members of the herd were all doing different things, one would say, "The herd of elephants are drinking at the waterhole, milling around, and resting in the shade."
I believe this is true for all nouns that encompass a group, e.g., committ