The clip has now been viewed nearly seven million times and has sparked a huge debate online, with commenters branding their actions "sad".
In a clip called "I Let My Best Friend Have Sex With My Boyfriend", Lena is seen preparing for a threesome with her friend as they sit on her bed in their underwear.
Lena and Emily admit they've had a threesome together before and say they want to
"share" Adam's penis in the X-rated and expletive-laden clip.
"I've been seeing Adam for a few months and we are both sexually pretty open to having other girls join, it's kind of weird and controversial to have your best friend join,"' Lena said.
The pair can be seen getting ready for the tryst, by drinking tequila and listening to music.
Adam then arrives at their apartment and admits he's just as surprised about the threesome as viewers are.
"Let me just say, I can't even wrap my f****** head around how my life got to the point where I'm engaging in - not only this activity in the first place - but with two girls that look like this," he says to the camera.
The pair switch the camera off to have their threesome and are later seen rolling around on Lena's unmade bed.
Viewers were very critical of the raunchy clip, with one commenting: "It's so sad how little girls turn into these things... if you ever have a daughter, teach her and protect her, don't let your angel turn into these... these animals..."
Another said: "You guys are attractive but I find it sad that only sex, drugs, alcohol and aesthetics are the only things that matter to you."
"This is actually allowed on YouTube?" a third posted.
Lena describes her videos as 'daily stories, naughty features and occasional direct chat', and earlier this year she sparked controversy when she promised to release a sex tape if she got to one million followers on YouTube.
Watch the video here - Warning: video contains explicit language
