Summoned Creature Initiative

My players and I recently came to a situation where a player used summon berserkers. A question came up of whether the Berserkers are rolled into the current initiative or wait until the following initiative roll. Any help?

Creatures which require concentration by the caster directly (elementals) act on the caster’s initiative. Other creatures act on their own initiative.

The berserkers begin acting the round after they are summoned.