Taylor Swift -- I'm With The DJ

Taylor Swift kept her man within arms reach Friday night while she partied away with her gal pals.

Swift was at Omnia Nightclub in Vegas to celebrate bff Karlie Kloss' twin sisters' bday ... and it just so happened boyfriend Calvin Harris had a gig there the same night..

We're told members of Haim joined Taylor and so did her brother Austin ... they all chilled in the DJ booth.

You know what they say ... the couple that clubs together ...