Is there a way to configure a User Input task to send Email to the task assignee

That's correct:

1. If the assignment ends up being set to a specific user (e.g. through a pv!) then the user will get a: "New Task Assigned to You" e-mail. It will be the e-mail address in the user profile.

2. If the assignment is for a group each user within the group will get a "New Task Assigned to Your and Others" e-mail for anybody to accept it. Once you accept it you don't get another e-mail, only one when the task is assigned.

For this to work make sure you've configured Appian to send e-mails (See Sending E-mails at forum.appian.com/.../Mail_Server_Setup) and that the alerts haven't been disabled by the user
forum.appian.com/.../Notifications forum.appian.com/.../Creating_Notification_Rules nor on the server forum.appian.com/.../Configuring_Notifications

That's correct:

1. If the assignment ends up being set to a specific user (e.g. through a pv!) then the user will get a: "New Task Assigned to You" e-mail. It will be the e-mail address in the user profile.

2. If the assignment is for a group each user within the group will get a "New Task Assigned to Your and Others" e-mail for anybody to accept it. Once you accept it you don't get another e-mail, only one when the task is assigned.

For this to work make sure you've configured Appian to send e-mails (See Sending E-mails at forum.appian.com/.../Mail_Server_Setup) and that the alerts haven't been disabled by the user
forum.appian.com/.../Notifications forum.appian.com/.../Creating_Notification_Rules nor on the server forum.appian.com/.../Configuring_Notifications