Hi @Kathleen_Spangler and welcome!
You probably won’t be surprised to know that many other folks want these two features as well. Here are existing product requests for them - the best thing you can do is add your vote! You can also read more details about why they aren’t currently available if interested.
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Allow multiple people to be assigned to a task
See Assign multiple assignees on one task -
Allow the Fields to be applied to subtasks within a project
See Custom fields needed on subtasks