Learn Spanish (Latin America): Los expertos a quienes les mostramos el proyecto conocen el sector. - The experts to whom we showed the project know the sector.

We need the a because the experts are the indirect object of the verb mostramos (we showed the project to them). When referring to people after a preposition like "to", "with", or "for", Spanish uses the preposition plus quien or quienes. Here, "to whom" translates exactly to a quienes.

Why is it "quienes" instead of "quien"?

Quienes is the plural form, and it must match the plural noun it refers back to, which is Los expertos. If we were talking about just one expert, it would be singular: El experto a quien le mostramos el proyecto.

Why is "les" in this sentence? Doesn't "a quienes" already mean "to whom"?

Yes, it does! However, Spanish uses redundant indirect object pronouns all the time. Even though we already established "to whom" with a quienes, Spanish grammar still requires the indirect object pronoun les (to them) before the verb mostramos. Think of it literally as: "The experts to whom we showed to them the project."

How do I know "mostramos" means "we showed" (past) and not "we show" (present)?

For regular -ar verbs, the nosotros (we) form is spelled and pronounced exactly the same in both the present tense and the preterite (simple past) tense. You have to rely entirely on context to know which one it is. Here, the English translation confirms it happened in the past.