Learn Spanish (Latin America): El que ellos conozcan el método es una gran ventaja. - The fact that they know the method is a great advantage.

While you can start a sentence with Que to mean 'That...', adding El makes it sound more natural and noun-like, translating closely to 'The fact that...'. It packages the whole idea into the subject of the sentence. Without El, Que ellos conozcan el método... is grammatically correct but sounds a bit more formal or literary.

Why is conozcan in the subjunctive here?

The subjunctive is triggered by the phrase es una gran ventaja. When a clause is the subject of an evaluative statement (giving an opinion, judgment, or emotional reaction like 'it is a great advantage'), the verb in that subject clause must be in the subjunctive.

Is the pronoun ellos strictly necessary, or could I just say El que conozcan...?

You can absolutely drop ellos. El que conozcan el método is perfectly natural. Including ellos just adds emphasis or clarifies who you are talking about if it isn't already obvious from the conversation's context.

Why is it gran ventaja instead of grande ventaja?

The adjective grande drops its final syllable and becomes gran whenever it is placed directly before a singular noun, regardless of whether that noun is masculine or feminine. Placing it before the noun also gives it a more figurative meaning ('great') rather than literal size ('big').