Legality of reimplementing an app for both education AND personal use

Hi guys, I am looking to reimplement a rock climbing app called Stokt in react native to build my personal portfolio, however its also partly because the relatively simple service they provide is paid. I'm going to assume if I only use my implementation myself there's no issues there but what's the legality of me providing it to a few friends for their use? I don't intend to sell or widely distribute this at all but I am still worried that the company may somehow find out as climbing specific software is still in its infancy and if people see I made a free version of a semi popular app I'm worried word will get around. I only seek to implement the base features of this app and not the frills like the social networking aspect. Sorry for being vague, if you want to know what I am planning to do you can just go to my old post history on r/learnprogramming but I don't really need to explain further here.

Also would this be a case where I could leverage friends using my software as a "userbase" if asked or is that unethical because I'm basically using someone else's idea?