This may happen if you are using the "Installed user"
option. When our "Select an account" dialog picker is displayed the user group entries from "User and Groups" page are not displayed. Only the user account entries are displayed. This is an Advanced Installer limitation, thank you for bringing it to our attention. We will fix it in a future version of Advanced Installer.
However, please note that we do support for scheduled tasks to configure them to run under user groups. You should just use the "Property based user"
and manually typing the user group name.
Let us know if this helped.
All the best,