I and one of my friend plan to fly mavic mini side by side. As both uses WiFi on 5.8 signal, will it affect our mavic mini?   Will there be any signal interference ?

If you stand too close to each other then yes.

Keep a reasonable distance apart and you should be OK.

One of my friends was facing the same issue but got fixed after using some of Wifi Extenders, do let me know if it works for you too.