Maghreb praying time

Asalamualaykum,

I am in a land that is very cloudy, and extremely light-polluted. My eye sight is also a bit weak. I want to ask, is there any other way to determine maghrib prayer time for prayer or for breaking fast? Can I just use the Sunni or the twelver prayer time? A group of my Sunni friends invite me to pray in congregation with them however they pray right after their athan my a few minutes (around 5). Is it permissible for me to pray with them during that time?

The sign of the entry of night is the appearance of darkness from the eastern direction, the appearance of light on the face of the moon, and the sighting of a star in the northern half of the sky. If any one of these is observed, then night has entered. 

علامة دخول الليل أن يظهر السواد من جهة المشرق، وظهور نور وجه القمر، ورؤية كوكب من النصف الشمالي للسماء. فإذا وُجد واحد من هذا فقد دخل الليل.

- Sayyid Abdullah Muhammad Awad

The Sheikh means the Time of Maghrib by 'Night'.

Therefore you can observe the eastern direction to see if the Red twilight has disappeared and the sky has become somewhat dark.