Learn Spanish (Latin America): Estoy en la reunión contigo.

In Spanish, when you want to say "with you" (informal singular), you cannot just put con (with) and tú (you) together. Instead, they combine into the special, single word contigo. It is an irregular form you just have to memorize! (Similarly, "with me" is conmigo).

Why do we use estoy here instead of soy?

We use estoy (from the verb estar) because the sentence is talking about your physical location. A great rule of thumb to remember is that for locations, like being in a meeting, at home, or in a city, you always use estar, not ser.

Does the word en mean "in" or "at" here?

It means both! In Spanish, the preposition en is very versatile and covers both "in" and "at" (and sometimes "on"). So whether you would say "I am in the meeting" or "I am at the meeting" in English, Spanish just uses en la reunión.

How do I know that reunión uses la instead of el?