I really enjoyed using Ohana. I consider it safer than FB groups because it is a third party between you and the sublessor. Everything is documented on the site (pictures of the place before and after, submission of payments/deposit, contract between you and sublessor) so if anything goes south, Ohana will help you. There's a small fee they take out of your initial deposit but I consider this worth it knowing my deposit is safe and the reasons above. They don't have an app yet only a website but they are legitimate, me and my friend have used this site and had no problems. I can show proof too if you want to reach out to me separately.
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