It's not a typo; it's the definite article meaning "the".
In this sentence the subject is "el veros a todos aquí": the head noun is the infinitive "ver", which is accompanied by its direct object, "os a todos", and an adverb, "aquí".
In English we would use the gerund "seeing" as a noun in contexts like your sentence, while Spanish uses the infinitive "ver" as a noun.
English typically does not allow using an article with a gerund in this type of context. However, Spanish frequently allows using an article with an infinitive in this type of context, and sometimes Spanish requires using an article.
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