What sort of jazz do they play? The style of solos should fit in with this style to an extent. You could study chords and/or modes and base your solos on these, or you could just play whatever you feel like at the time. This is easier said than done, of course. When we speak we unconsciously draw on our knowledge of words and sentences so when we play whatever we feel like playing we need a lot of the equivalent of words to hand. The notes and phrases must flow easily. This takes a lot of practice and studying.