Just in this sentence means rather than doing something else in addition. Question: Do you want to get together for dinner before the match? Answer: Let’s just meet at the stadium before the match around 7:30. The answer means No, I’d rather not do anything else in addition to meeting around 7:30.

In this context, 'just' means 'simply'. "Let's simply meet at the stadium".

"Just" refers to a limited selection or choice. So in this sentence it means that the person can't meet a long time before the sports game starts (perhaps he/she is busy during the day), so suggests to meet shortly before the game starts.