Hi Beni,
well, the multi-value field is being represented as a JSON disctionary in the workflow, which looks like this:
{"__metadata":{"type":"Collection(Edm.Int32)"},"results":[11,13]}
In order to assign each person permissions, you first have to extract the peoples and then loop over the list like Warwick Ward already showed. So my workflow looks like this:

At first I write the value of the multi-value-person-field into a dictionary called "varDictPersons".

Remember the JSON array above? So next we need the results, which is actually another array. So we want to store the results in a collection, in order to loop over them.

Now that we have a collection, we can just simply iterate over the items and assign them to a variable "varCurrentPerson".

It might be worth mentioning, that "varCurrentPerson" is of type "Person". This way you'll get a valid sharepoint user object, without needing to explicit call any web-service which would turn the user-id into an user-object.
Finally you can assign permissions to this user!
Voila!