When suffixing the first-person plural reflexive pronoun to the imperative, the 's' is dropped from the first-person plural ending.
A similar rule exists for the second-person imperative of a pronominal verb, only it is the 'd' that is dropped.
This is explained
here, for example.
levantar (imperativo)
| 1st Person Plural | 2nd Person Plural |
|---|
| levantemos | levantad |
levantarse (imperativo)
| 1st Person Plural | 2nd Person Plural |
|---|
| levantémonos | levantaos |