What’s the word to describe a person in this role?

Solution 1:

Possibilities include:

• a secret accomplice
• a secret confederate
• a secret co-conspirator
• a shill
• a plant
• a ringer

You could use hidden or unknown or some such in lieu of secret. The entries without secret already contain that element built right into them.

Solution 2:

The first word that comes to mind for me is

"a stooge [in the audience]" - any underling, assistant, or accomplice.

There are lots of alternatives. I've linked there to synonyms for shill, but I think that's more strongly associated with fake buyers (in an auction, for example), rather than accomplices in a prank. The best of the rest is probably plain and simple plant.