The Spanish preposition con means "with". Let´s review some of its common uses:

Yo quiero ir al partido de fútbol con ellos.
I want to go to the soccer game with them.

Trabajo con los caballos.
I work with horses.

Hablé con el director.
I spoke with the principal.

¿Quieres ir al partido de fútbol con nosotros?
Do you want to go to the soccer game with us?

Yo iré al partido de fútbol con ustedes mañana.
I will go to the soccer game with you all tomorrow.

However, you cannot use con in the same way to say "with me" and "with you" It is a special case with the following pattern.

with me  - conmigo      with us             - con nosotros
with you - contigo      with you all        - con ustedes
with him - con él       with them (males)   - con ellos
with her - con ella     with them (females) - con ellas

Correct Use
conmigo - with me

contigo - with you

Incorrect Use!
con yo
con tú

Let's look at a few examples:
¿Quieres venir conmigo?
Do you want to come with me?

Venga conmigo.
Come with me.

¿Quiénes irán contigo?
Who is going with you?

Yo no puedo ir contigo.
I can't go with you.