Simple question Where do I control who is on the list you can choose as "from Name" when you sent an email?
It seems like its some of the User Profiles for the account, but not all of them.
The From Names and From Emails in this list come from the Name and Reply Email Address values of the users who are assigned to the Business Unit you are currently in.
The ability to see this list of emails (as opposed to just the From Name and From Email of your specific user) is controlled by a Business Rule that Support has to turn on for you. Unfortunately I'm not sure if this BR requires add'l payment or not.
You can see which users are assigned to your current Business Unit by going to Email > Admin > Business Units, then click on the checkbox of a BU and hit View Users.
Those Email address are coming from, which are under Org-wide Adresses('Email Administration'-->'Organization-Wide Email Addresses') and verified Email-to-case addresses