You may not light Shabbat (or Jewish festival) candles before plag ha'mincha. The candles must burn until at least dark (tzeit ha'kochavim) and someone must be there to see the light from the candles after dark.
SITUATION Mincha minyan begins at plag ha'mincha. You cannot light candles at home and still get to mincha minyan on time.
WHAT TO DO You must say mincha on your own (anytime from half an hour after mid-day until sunset). You will light candles after plag ha'mincha but before sunset and not join the mincha minyan.