I didnt read the rest of the posts but thwat you need to do is play with them creating an odd number and shooting for teams.
If the all the shooting you had done previously is anything to you or the other people there should be a mixed result and you should be able to play in the 5v5.
Where im from every1 at the court plays. So if you were there shooting alone and a ggroup came along and wated to play full court you need to join them.
Otherwise they will say **** you for not playing with us we will play through you.
If they dont let you join thats a different story which you dont see to much. If they dont let you join keep shooting and interrupt their game. You were their first and its their fault for not letting you play that their game has to be interrupted.