<p>Ah, I see. But I thought it was the second one since when the subject contains a pronoun that differs in person from a noun, the verb must agree with the closer subject word.</p>
<p>You’re combining ideas that could be expressed in two sentences.*Don is leading the discussion. You are leading the discussion. You could apply the verb to either part of the subject. You don’t apply the verb to both parts of the subject at once. Also, saying “you or Don” would probably make it easier to understand.</p>
<p>The second one with “you are” is correct. In an either or subject phrase the verb needs to agree with the second noun used. In the sentence that is you and you always takes “are” regardless of whether you is singular or plural.</p>
<p>When you are using either or neither you have to make sure that the verb corresponds to the noun closest to it. In this case the noun is singular so you have to have is rather than are so the first sentence is correct.</p>
<p>The first one is correct. When the coordinating conjunction OR is used in a sentence, the verb goes with the closest noun or pronoun. </p>
<p>From the online GUIDE TO GRAMMAR AND WRITING:</p>
<p>"The conjunction or does not conjoin (as and does): when nor or or is used the subject closer to the verb determines the number of the verb. Whether the subject comes before or after the verb doesn’t matter; the proximity determines the number.</p>
<p>“Either my father or my brothers are going to sell the house.
Neither my brothers nor my father is going to sell the house.
Are either my brothers or my father responsible?
Is either my father or my brothers responsible?”</p>
<p>If you want to confirm it, just do a Google search for coordinating conjunctions and verbs.</p>