Expression rule for Email Address

There's a lot of detail left out here (how your users are set up, etc), but in general, if you start with an Appian username, you can get their email address (in plaintext) by using the "user()" function, i.e. user("myUsername", "email") (where "myUsername" is any given user's actual username).  Note that you need to wrap any plaintext email address(es) in "toEmailAddress()" before it'll work in the Send Email node; alternatively, you can set the "to" field to point to an entire Appian Group and/or actual Appian Users and it will generally just work.