Can you use convoke to tap creatures that just entered the battlefield this turn?

Convoke (CR 702.51) is a static ability that modifies how you pay for a spell. When you cast a spell with convoke, you may tap any number of untapped creatures you control as an additional cost. Each tapped creature reduces the spell's cost by one mana of any color that creature could produce, or by one generic mana.

The crucial point is that summoning sickness (CR 302.6) only prevents a creature from attacking or using activated abilities with the tap symbol ({T}) in the cost. Tapping a creature as a cost to cast a spell — which is what convoke does — is not the same as using an activated ability. Therefore, summoning sickness does not prevent a creature from being tapped for convoke, even if it arrived this turn.

This is confirmed by CR 702.51c, which states that tapping a creature for convoke is an additional cost and is not subject to the normal restrictions of summoning sickness, because convoke is a cost applied during the casting process, not an activated ability of the creature itself.

Example: You attack with no creatures, then cast a token maker that puts a 1/1 Goblin onto the battlefield. On the same turn, you want to cast Chord of Calling (which has convoke). You can legally tap that brand-new Goblin token to help pay for Chord of Calling, despite it having summoning sickness.

So if you control several creatures that just entered the battlefield, you can tap all of them for convoke — just remember they still cannot attack or use {T} activated abilities that same turn.