Learn French: Vous jouez au tennis avec eux.

Because the subject is vous (you). For regular -er verbs like jouer (to play), you drop the -er and add -ez when the subject is vous. The -ez ending sounds just like the letter 'A' in English.

Why does it say au tennis instead of just jouez tennis?

In French, when you talk about playing a sport or a game, you must use the preposition à (at/to) followed by the word 'the' (le, la, or les). Because tennis is a masculine word, à and le squeeze together to make au. The French literally say 'play at the tennis'.

Why is the word for 'them' eux instead of ils?

Ils is a subject pronoun, which you only use when 'they' are doing the action (like 'they play'). When 'them' comes after a preposition like avec (with), you must use a stress pronoun. The stress pronoun for a masculine or mixed group is eux.

Do I pronounce the 's' at the end of Vous here?