I'm developing a Windows 8.1 app and I need to send it to somebody for testing. That somebody only has a windows 8.1 tablet, without admin rights, and they can't sideload it apparently. Any idea how I could do that?
You could upload it to the Store and lock it using a password that only the tester knows. I don't really recommend this as you would have to give the testers permission to use it also.
Otherwise there's no magic that will put an app on a machine without sideloading. Buy this tester another machine.