Either can be possible, "přátelé" or "přáteli". To be someone can be translated into Czech with nominative or with instrumental. There is a very fine distinction and in some cases even most native speakers don't know it.
In this case "přátelé" is a noun so the distinction says that you use the instrumental case if the validity of the sentence is temporary, if it is just currently valid, and you use the nominative case if the validity of the sentence is general or perpetual. But you can hear both, this distinction is not always observed.
References in Czech:
A long article: http://nase-rec.ujc.cas.cz/archiv.php?art=6204
An older long article: http://nase-rec.ujc.cas.cz/archiv.php?art=1903