Yandex is a Russian search engine which also provides personal services like email, address books and calendars.
Choose "Login with email address" and enter your Yandex email address and your password.
Contact group method: groups are per-contact categories
Only the group "Phone contacts" in the Yandex address book will be used for CardDAV (i.e. be synchronized with DAVdroid)! You can assign contacts to other groups and "Phone contacts".