Assigning a task to multiple team members

In the standard workflows you can't assign a task to multiple owners.

You might create a (fixed) subteams and select a subteam with the filed against attribute.

i agree with @hajo62, standard workflows do not encourage multiple owners...
infact there is a saying like "multiple ownership means no ownership"... that was for the lighter side...

having said that, to be able to have more owners, I would suggest splitting the task and making them sub tasks of a main task.
then one person responsible for a certain action becomes owner of that specific task under the main umbrella task. 
the main umbrella task itself can be owned by a single lead/responsible, who is in-charge of the overall delivery