Learn Hardcore German: Außer mir kam niemand pünktlich zur Besprechung. - Except for me, nobody came to the meeting on time.

Besprechung: general “meeting/discussion,” neutral in tone.
Sitzung: a formal session/meeting (committees, councils, boards).
Meeting: English loanword, widely used in business; informal to neutral. All can be appropriate depending on context; here Besprechung is a safe, neutral choice.

Because außer is a preposition that governs the dative case. The personal pronoun for “I” in the dative is mir (ich = nominative, mich = accusative, mir = dative). So: außer mir = “except for me.”

Can I say Außer ich instead of Außer mir?

In standard German after the preposition außer, you should use the dative: außer mir. You will hear sentences like “Alle kamen, außer ich” in colloquial speech (an elliptical “except I [came]”), but in your sentence, starting the clause with “Außer ich kam …” is widely viewed as nonstandard. Prefer:

Außer mir kam niemand …
Niemand außer mir kam …
Nur ich kam …

Why does the verb come before niemand (Außer mir kam niemand …)?

German main clauses follow the V2 rule: the finite verb must be in second position. When you front something (here, the prepositional phrase Außer mir) into the first slot, the verb (kam) takes the second slot, and the subject (niemand) follows it.

Why is it kam (singular) and not kamen (plural)?

Because niemand (“nobody”) is grammatically singular. The verb must therefore be singular: niemand kam, not “kamen.”