I'm creating some forms to internal company use. I'd like to list the usernames just like outlook does, after typing the beginning of the username i'd like to have the suggestions of the names. Furthermore, I'd like to send an custom email to the user that was selected.
You can use SharePoint client picker control in your form. It does have functionalities to show suggestions. You can write anything i.e. User Name, Display Name or email of user and it will show suggestions based on input.
Hope it will help to you.