If you want to know how to know wehter it is the one 'zij' ('she') or the other 'zij' ('they'): this is usually clear from context.
If you want to know what this 'drinkt'/'drinken' business is all about, read Grammar: The Dutch present simple. The Dutch conjugate their verbs. This means that the verb changes with who 'executes' the verb. Like "I know" vs. "he knows", only with more verb forms.