The Gmail app for Android recovers a handy feature that already existed on the web version of the email service. In this case, it is the shortcut "+" or "@" which allows you to quickly add people in the recipient field when you are filling in the body of the message.
All you have to do is use one of these special characters and add the first letters of a name to automatically open a drop-down menu with choices from the address book. It will then suffice to validate the right person and presto, it is now one of the recipients. To remove it, it will then suffice to delete the special character.
Detected by 9to5Google, this update is already available to G Suite customers and should soon be available to all other Gmail app users.