Adding Responsibility - Users or Groups

There are two fields in my form where individuals can select either users or groups to be assigned as stewards of a business term. These are two separate dropdowns - one for the user and one for the group. How can I add the users/groups to a specific responsibility?

If the user field is populated, the users should be added as stewards. If the group field is populated, the groups should be added as stewards. However, both fields cannot be populated at the same time. If both fields are filled, an exception should be thrown. How can I code this?

Thank you!