How to participate in clan war instead of being a spectator

The participants in a war are by multiples of 5. If a clan has 19 people whith greenlight for war, only 15 people will actually participate and the 4 others will just watch. I am not aware though on which criteria each participants are selected.

You have probably started the clan in the middle of a battle, meaning you have to wait this one out. However it may be that your leader decided he didnt want you in the war, which he can choose at will.