Learn Spanish (Latin America): Los fundadores a quienes se lo envié son nuestros socios. - The founders to whom I sent it are our partners.

Quienes is the plural form of quien (who/whom). We use the plural here because it refers back to los fundadores (the founders). When referring to people after a preposition like a, you use quien for one person and quienes for more than one.

In everyday English, we often say "The founders who I sent it to." Can we move the a to the end of the clause in Spanish?

No, you can never end a sentence or clause with a preposition in Spanish. The preposition (in this case, a) must always come right before the relative pronoun. That is why the structure matches the formal English "to whom" (a quienes).

Why is it se lo envié instead of les lo envié?

In this sentence, you are saying "I sent it to them." The "it" is lo and the "to them" is normally les (referring to the founders). However, whenever the indirect object pronoun le or les comes right before lo, la, los, or las, it changes to se to make it easier to pronounce. So, les lo becomes se lo.

Can socios also mean romantic partners?