Are humans Summonable?

In this comment, the question was raised: Are humans Summonable?

In the Triwizard Tournament, Harry Potter summons his broom.

In the second task, none of the contestants even attempt to summon the hostages. Is this because a human is un-Summonable?

I don't have my book right now, but Harry Potter attempts to Summon Rubeus Hagrid in the Battle of Seven Potters. However, as Hagrid did not come shooting towards him, the spell didn't work. But, Hagrid is half-giant.

We also see, in The Goblet of Fire, that Harry Summoned a fly. (Although it is mentioned that maybe the fly was just stupid, and that maybe the spell didn't work, and that it was a coincidence.)

Does 'Accio' work on a human being?