Google's new messaging app, Allo, is not by any means a comprehensive chat solution. It is, however, an interesting look at a number of new features that you may not typically find on a run of the mill app like Facebook Messenger, LINE, or WhatsApp. While the headline feature, Google Assistant capability, is still strictly in Allo's territory, at least one of the app's oddball features is going to be opened up to developers of other messaging apps. That feature is called "App Preview Messaging", and allows a user to send messages to anybody in their contacts, via Allo, without them having the app installed, so long as the recipient is using an Android phone; the feature depends on Google Play Services.
When a user sends a message to somebody who does not have Allo and is using an Android device with Google Play Services installed, they'll receive a standard quick reply prompt, along with a few extra options. They can ignore the message entirely, send a quick reply, or install the app that the message came from. Obviously, since the recipient does not already have the app that the message originated from, ignoring the message or simply swiping the notification away means that the message is lost forever. According to a sample picture that Google produced with their blog posting, the user will be able to see what messaging app the message originated from, but it's unknown if they will see the message again if they choose to install the app in question later on.
Google is opening up the feature to developers of other messaging apps on a limited basis, according to an announcement on their developer blog. Google will be opening up the feature on a limited testing basis, with more information on how to participate in the preview of the feature hitting their blog at a later date. Google did not state exactly how long it would be until they actually open up the feature, how long it will be in a testing phase, or how many developers they will allow to sign up and use the feature.