If you have the Windows Phone SDK installed, and have a developer account, you can run the "Windows Phone Developer Registration" program (Search for the title in quotes in your start screen).
There are two ways to get your program on the phone from this point. The first is to connect your phone to your PC with Visual Studio on it, turn on and unlock your ...
Follow these steps:
Go to "All settings"
scroll down to "Update & security"
you will find tree options: 'Windows Store apps' - 'Sideload apps' - 'Developer mode'
Choose the one that fits you
Adding to Omar's answer:
You'll need the "Sideload apps" or "Developer mode" setting, of course. Developer mode gives you more abilities but also puts your phone at greater risk of security issues and may consume more of its resources.
Once you have those, you have to get the apps onto the phone. For either setting, you can copy the .xap/.appx/.appxbundle ...
As for the latest build, you can sideload n number of applications. There's no limit.
If you want to sideload/deploy apps to your phone, use 8.1sdk or xap deployer 2.0 or wppt on pc to do it. Also assure you install ipoverusb in pc sothat app deployment over usb is possible! 8.1sdk works with win 8 and 8.1pc but xap deployer 2.0 and wppt needs win 10 with 8....
There is an easier way than what has been desdcribed to you, I own a Microsoft Lumia 435 and I was able to do this with an app I had developed.
Go to Settings → For Developers, and enable the "Sideload Apps" option.
Now you need to plug your Lumia into a USB port on your computer/laptop and copy the .xap file from your computer to your Lumia's SD card....
The easiest way would be to publish it to the store as a hidden app and provide your friends with promotion codes so only they can find & download it.
Afaik you need the developer tools to unlock your phone for sideloading otherwise.