Playing in tune in an ensemble involves listening and matching pitch with octaves and unisons, tuning out "beats" with 4ths and 5ths and making the other intervals 3rds, 6ths, 7ths sound right. An exercise that helps to develop the facility to "humor" the pitch up or down and to hear the relationships between notes is to play a unison with another player or fixed pitch instrument. First you lip your note above the other pitch, then lip it below, and finally lip the note to perfectly match the other pitch. When this is done well, it is hard to distinguish whether there is one or two instruments playing---especially if you match not only the pitch, but the timbre, and dynamic level as well. Playing all of the notes of a scale against a fixed pitch drone is a good way to hear and tune all of the intervals.