First you have to create a community or be invited to a community. As soon as you are a member of a community, you have to create and save your team for the matchday as usual. Then you have to click on "PLAY" and select the pitch you want to participate in. Only then will you take part in the matchday. It is not enough to be in a community to participate.
Currently you can only use the Mobile App on Apple devices. We are working on offering SPITCH on Apple devices for Mobile-Web as well.