Question

What is the regular ending for an -ar verb in the "ustedes" form?

Answer 1

But if your talking about -Ar verbs in Preterite tense then it's

Example: Jugar = drop the -ar and add -aron which equals Jugaron

Answer 2

If we are talking about just plain old present tense, then it is -an.

Example: hablar
ustedes hablan