One of the required fields while creating an objective is the owner, the owner is the individual responsible for making sure the objective progress and assessment are up to date, although the owner is the main responsible for the objective the owner doesn’t usually do all of the work. They need support from their team, peers, or boss in order to succeed. Those are contributors.
When you create a new Objective or edit a current one, you’ll find contributors in the top right hand corner. You can select an entire team, or specific members to join on and contribute to your objectives.