There are a few ways to achieve this without getting the certificate required for sideloading.
Beta
First of all - you ask for the simplest way to distribute an app.
I assume that there might be updates in the future (and if only to fix this annoying little bug that might find or whatever). Then submitting it to the store as Beta app is really something you should consider; it won't be visible - it won't be accessible to anyone whom you haven't authorised to use the app. The certification process in Beta is quite fast, as it is merely a sanity check.
Regarding the cost of the dev account: it is quite cheap (https://msdn.microsoft.com/en-us/library/windows/apps/jj863494.aspx) and can even be free, for students and startups through the respective programs (DreamSpark, BizSpark). Also: no recurring fees, even if you're not a student anymore.
But the true benefit is that all you need to do is give the authorised testers the link to your app once it is published and then all the updates are handled through the store. No messy package distribution to the individual people, no notifications - testers will get the updates like for every other app. Long term, that would save you a lot of time and the testers a lot of hassle in preparing their device for being able to play your app, they do not need to developer unlock their phones.
Without Beta submission
If you want to just send them the app package,
they must (also) have a Microsoft account. If I recall correctly, they can have one developer unlocked device even without the dev registration (http://dev.windows.com/en-us/join)
they need the Windows Phone SDK, which comes as part of Visual Studio, which installs the required tools
connect their phone to the computer
to developer unlock their device, they have to run a tool called "Windows Phone Developer Registration" and sign in with their Microsoft account
to deploy the app, use the tool called "Application Deployment".
Everytime an update is distributed, they simply need to use the "Application Deployment" tool.
or use the command line :D