How do I write migrate group files?
I believe that groups are a feature of the migrate_plus module. In order for them to be recognised, the file needs to be named migrate_plus.migration_group.&lt;group_name&gt;.yml. The actual migrations also need to be formatted like this migrate_plus.migration.&lt;migration_name&gt;.yml.

One other issue I see is that your id: wprdpress should be id: wordpress. (Just in case that typo is in your code :) )

The last thing is that migrations are only defined on module install. This means that you have to uninstall and re-install every time. (Or you can implement some drush commands to do it for you. Check out this tutorial.)