If your Community allows the inclusion of People to Notify, you will find this field on the post creation screen.
The section dedicated to People to Notify allows you to select other people directly involved in your post and generate an email notification to inform them about the creation of the post. Subsequently, interactions and changes to the post will also generate on-screen notifications for them to keep them updated on developments.
To add People to Notify:
click on the People to Notify box;
type the first letters of the people you want to add, then select them directly from the dropdown menu suggestions.
Once you've added them, you are ready to publish your post.