They are members of the Muhajireen Army, which is a foreign fighter faction in Syria led by a Chechen commander named Abu Omar al Chechen, they usually fight alongside al-Nusra Front.

Via LWJ:

The Muhajireen Army, a group of jihadists closely tied to al Qaeda’s affiliate in Syria, the Al Nusrah Front, claimed it shot down a Syrian Army helicopter in Aleppo using what appears to be an SAM-7 surface-to-air missile.

The claim, which is backed by a video [above], was made on FISyria, a Russian website that has been releasing propaganda for the Muhajireen Brigade. The video was published on the FISyria website as well as on YouTube. A fighter is seen shouldering the weapon, then firing on a helicopter, which does appear to have been hit.

At the end of the video the shooter says, in accented English, “I shoot it.”

The cameraman responses in nearly perfect English, “Really?”

“Yes, yes.” the shooter says.

“It’s falling down,” the cameraman then says.

According to a translation of the statement accompanying the posting, the group said that while the Free Syrian Army claimed credit for shooting down the helicopter, the FSA does not have the capability to do so.