The Skipper may define custom email groups to use in Skipper Send Email. The Manage Groups screen is available from the left menu in the WordPress Admin section. This is the same screen where the Skipper assigns Crew to the various groups (e.g. Retired, Core Crew, All Posts, Show Crew).
There is a section at the top of the screen that allows the Skipper to add, delete and change Custom Groups. Once added, the groups appear in a column with check boxes by each crew so they may be selectively added to the group. This picture below shows the new sections annotated.
Once added to the group, the Skipper Send Email page available from the Skipper menu for a logged in Skipper will include that group in the To: dropdown.