There are at least three different groups performing under the name of Cocktail:

1) Cocktail are a Thai group consisting of:
Puntapol Prasarnrajkit (Lead vocal ,Song writer)
Ayu Charuburana (Drums ,Loops ,Percussions)
Chawarat hansakunatai (Guitar ,String)

Youtube's Channel: http://www.youtube.com/cheerscocktail

Official Site: http://www.cheerscocktail.com

2) Cocktail was a Danish girl group made up of Lonnie Madsen and Tanja Jessen. After releasing two Danish-language albums, they changed their name to Zee-U in 2003, setting their sights on an older demographic.

3) Cocktail was a Swedish-produced dance-pop project that released one single, "Kakadu", in 2000.