Yes you can! Your recipient will receive an SMS notification of your payment, with a link to download the app. A Slide wallet linked to their information will be created on their behalf which will contain the funds owed to them, while they register using the details you used to send them money.
They will need to register the app in order to access the funds. You can also cancel the pending transfer at any time before that point, and the funds will be returned immediately to your Slide balance. They can take as long as they need, but once they register the money will be waiting for them in their brand new Slide wallet.