I think Romina is right. "Me dices si vamos a ir al café esta noche...." is a better translation. "Avísame" is a bit too imperative. Also.."join me" sounds a bit selfish for a Spanish speaker, like you make things about you.. That's the reason "vamos" is more appropriate. In latin cultures it's all about "us" not too much about "me" when it comes to socializing. And it will make the other Spanish speaker more comfortable. I hope this wasn't too long :) greetings!

"Let me know if you can join me at the coffehouse tonight" = Hazme saber si puedes reunirte conmigo esta noche en la cafetería. "Let me know" can be translated like this: - Hazme saber - Avísame

Hola Tom:"avísame" está perfecto; "hazme saber" también es correcto pero no es lo más frecuente. Saludos!

sí, avísame está bien