Thank You @fabioferrero for your quick reply.
Yeah, for uploading on App Store Apple Developer Enterprise Program can prove to be good option.
Similarly providing .APK with password just like PDF and .ZIP files can also be achieved , however in case of updating APK file it would be necessary to inform user and again send updated file.
This could be overhead when users are more in number.
Though I would like to get information if any other way where I can be able to provide password protection to app before downloading app so that app is made public on Play Store.
Thanks.