On Android, friends with Allo can bug you even if you haven't installed the app yet.

Update: Google has announced that it's opening up App Preview Messaging, which is the Play Services feature powering these notifications, to third-party developers. Allo is a great use case for this feature, but the documentation makes it clear Google sees uses for it beyond growing Allo's user base.

Google has built a unique trick into its new messaging app, Allo, which may help spread its reach throughout the vast Android user base. Allo can send messages to your Android friends, even if they don't have the app installed.

It'll look a little something like this:

Allo notification

Be prepared for confused responses from your friends.

The notification lets you respond through text along (as opposed to stickers, photos or anything like that), or alternatively ignore it altogether. There's also a button taking you straight to the Play Store install page for Allo.

How can Google do this? The notification is generated by Google Play Services, which is installed on just about every Android phone, and updates silently in the background.

For that reason, there's a good chance your Allo contact list will contain a good few people who don't have the app installed yet. And so be prepared for confusion when messaging people for the first time. The feature does seem to be a bit flaky, though. Some people without Allo installed who are on our contact lists aren't getting these "introductory" messages through Play Services, while others are.

Note: If you're messaging a contact who's not on Android, you may see a "free SMS" option, which lets you text them without paying the usual charges (by routing the message through Google.) However, it's not entirely clear why some contacts get the "free SMS" option, and others simply an "invite" button.

It's certainly a powerful way to drive installs of a new app, and we're interested to see what third-party developers make of this feature in future.

Anyone receiving (or sending!) random Allo messages this morning? Getting confused replies back? Shout out in the comments!