For 20 people, it might be just as easy to write down their BK numbers, and delete them one by one.
However, there is a way to delete the entire group.
First make a backup. (Keep this backup safe for a few days or longer.)
Then to delete a group, you use File, Split Database, and create a new folder name, then when it asks you to select people, first press <F3> (Function 3) on the keyboard and it will say "option to keep same BK code numbers."
Then select everyone with Range from 1 to the end.
Then pick the option near the bottom right to De-select people.
Then pick the group you do not want (and they will be removed from the selected number.)
Then pick Copy to copy the remaining people to the new folder.
When it is done, pick File, Open Database, and open your new folder. It will index the names, and then you should have everyone except for the people you de-selected.
Since the new database is in a new folder, you can either use that folder, OR you can Backup that one to a new place, and then open your old data folder and then Restore the new one over the old one.