No puedo entender esto

No puedo entender esto
 

Cepillense los dientes de acostarse.

Sé que tiene el sentido, "brush your teeth before you go to bed." but I'm having problems understanding why it's refering to the particular person the order is targeting I looked on the website, and cepillen comes up as ellos, not tú.

The third person plural ustedes also means 'you'. ;)

All of the following are possible translations of the English sentence you wrote (the person is not specific enough in English to give only one translation):
Tú (singular, familiar): Cepíllate los dientes ...
Usted (singular, formal): Cepíllese los dientes ...
Vosotros (plural, familiar): Cepillaos los dientes ...
Ustedes (plural, formal): Cepíllense los dientes ...

In Latin America, vosotros isn't used, so the plural of tú becomes ustedes, which is a third-person pronoun. In Spain, the plural of tú is vosotros.

Helpful?

So basically, it means, YOU ALL brush your teeth?

If you want to think of it that way, yes.

Technically, though, since this is a command, the subject is not said at all in English.
The command "Brush your teeth" is valid for addressing a single person as well as two or more persons.

The plural of 'you' is 'you'.

Thanks. I thought the book I am learning from is Spanish from Spain, but I now think it's American Spanish. This confused me loads.