What kind of bar is it? Some regular bars or pubs have
PR Boys or Promotion Boys promoting certain brands of beverages. Balcony and Telephone in Silom soi 4, for instance, have boys working for Tiger beer, for Heineken, and for San Miguel, among others. They mingle with the regular staff, and could easily be taken for one of them.
If it's at a go-go bar, it could be the person at the entrance trying to hail customers.