"I'm coming with you!"
In another thread, a commentor said that native Spanish users use the simple present more often than native English users do. That actually helped me during English conversations with native spanish speakers. Since we're now using the Spanish present progressive, Duo is helping us figure out when to use one or the other in Spanish.
Llegar is to arrive.
Estoy llegando contigo can be translated as: I am arriving with you
I said "estoy vengo contigo" and was marked wrong. Should i report it or do you must not say that?
No - you are using the same tense. If you are trying to say I am coming it would be estoy viniendo