Learn Spanish (Spain): Deja eso aquí.

Deja is the affirmative tú command (telling one person to do something) for the verb dejar (to leave). For regular -ar verbs, the informal command looks exactly like the he/she present tense form, ending in -a.

How would I say this to a group of friends using vosotros?

You would say Dejad eso aquí. To make an affirmative vosotros command in Spain, you simply take the full infinitive form of the verb (dejar), drop the final -r, and replace it with a -d to make dejad.

Why is 'that' translated as eso instead of ese or esa?

Eso is a neuter pronoun. We use it for 'that' when we are talking about an unidentified object or a general 'thing' whose gender isn't established yet. If you were talking about a specific masculine noun like a book, you would use ese, but for an unnamed 'that', you always use eso.

Doesn't the verb dejar sometimes mean 'to stop'?

Yes! When dejar is followed by the word de and an action verb, it means 'to stop' or 'to quit' (for example, deja de cantar means 'stop singing'). However, when it is used with a physical object or pronoun, like in this sentence, it simply means 'to leave' something behind.