Learn Spanish (Spain): Quien quiera venir, que venga. - Whoever wants to come, let them come.

We use the subjunctive quiera because the speaker does not have a specific person in mind. When referring to an unknown or hypothetical person (like "whoever"), Spanish uses the subjunctive. If the speaker were talking about a specific, known person who wants to come, they would use the indicative quiere.

Can I say Cualquiera que instead of just Quien to mean "Whoever"?

Yes, Cualquiera que quiera venir is also completely correct and means the exact same thing. However, using just Quien followed by the subjunctive is a very common, concise, and natural way to say "Whoever" in Spanish.

What is the grammar behind the phrase que venga?

This is an indirect command, sometimes called a third-person imperative. By using que followed by the present subjunctive venga, you are expressing a command, permission, or wish directed at a third party. It translates directly to "let them come" or "have them come".

How would this sentence change if I specifically meant "Whoever" as a plural group of people?

If you are specifically thinking of a plural group, you should use the plural pronoun quienes and make the verbs plural to match. The sentence would become Quienes quieran venir, que vengan.