Yo me apunto

How do you say Count me in in Spanish?

Yo, sí often equates with the response I do. — ¿Quién quiere helado? — Yo, sí. Who wants ice cream? — I do. Tú no te diste cuenta, pero yo sí. You didn't realize it, but I did.

Yo me apunto

The verb apuntar is sometimes used with a similar function. — ¿Quién se apunta? Who's in? Who's up for this? — Yo me apunto. Count me in. Entrar, with le, is used similarly. Yo le entro. I'm up for it.

Emphatic sí

In English we often use intonation to convey contrast: Well I don't know about you, but I go to the gym five times a week. You can imagine the speaker here raising their voice on I, creating a contrast with you.

By constrast, Spanish intonation is flatter, with fewer highs and lows. The kind of emphasis indicated in English via intonation is often accomplished in Spanish by including a personal pronoun and/or sí. Yo sí tengo vida social. I have a social life (as opposed to the person the speaker is talking about, who is claimed to lack one).