Translation:Let's play at the sports field.
아/어요 is a multi-purpose ending. It can be used for 1st, 2nd, and 3rd person, both singular and plural, and can also be used as an imperative or a suggestion. 가요 often is best translated "Let's go."
Korean is an extremely contextual language, which is why, like a broken record, I keep suggesting DL do something to add context to these examples.
"at" is also correct, but has a different nuance from "on" or "in," both of which would probably be more common. "at" implies a different location from where the speaker is currently located.
A: "Do you want to play ball here in the street?"
B: "I always get scraped up on the asphalt. Let's play at the schoolyard/playground."
Of course, we could also say, "Let's play on the sports field at the school."