how make the background black

The biggest problems here are the way you placed and lit your subjects.

Your subjects are so close to the background that subject lighting is making the background gray, not black, and the subjects are casting shadows on the background.

First, you need to keep subject a minimum of 6' from the background to keep their shadows off the background.

Second, you should have lit the subjects in such a way that as little as possible light hit the background so that it would be black.  Depending on how you light and what type of light modifiers you use you might have to flag the background to keep the subject light off of it.

A hair light would have helped separate the middle subject's dark hair from the background.

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Living and loving it in Bangkok, Thailand. Canon 7D - See the gear list for the rest.

Thanks for the suggestion Sailor Blue. I just didn't have enough room (and background) to move the subjects further from the background
I don't really mind if the background is grey due to the vicinity
Good point with the hair light will keep that in mind