Distribute the Android application via Google Play and as an .apk file directly

I created an application that we will sell to customers through the Google Play store.

We also have people in the house who will support external customers and also use it in the house. They must have an application running on their own device. If I transfer them to them through an .apk file, will they be able to receive updates via Google Play? Or should they purchase it through Google Play for updates?

Secondary question: is there a better solution for distribution among internal users?