Answer:
Of course, because we care for everyone and we are not partial. JW's show not only love to fellow brothers & sisters in the congregation, but to non JW's as well. This was bought out in Matthew 22:37 37 "'You must love Jehovah your God with your whole heart and with your whole soul and with your whole mind.' This is the greatest and first commandment. The second, like it, is this, 'You must love your neighbor as yourself.'" This is what is well known as the Golden Rule.
We also take note if we only prayed or paid attention to ones within the congregation we would not be doing any extraordinary thing as Jesus pointed out in Matthew 5:43-38. This is one of the reasons of the door-to-door work that JW's conduct, is the love of all persons.
Of course, we even encourage it.
We can absolutely pray with people that are not JW's like the person said before me we show love to everyone. JW's are kind people and don't think anyone is better than others. We preach to people and when the study is done we pray with them. It's a way we show that we actually care about them. It really doesn't matter who prays, god listens to every one if it is a good intention!!!