I was chosen for an action, but when I go to join it says it's full

Unfortunately, if the action disappeared or you get a “this action is full or not targeted to you” message when you go to join, that means that by the time you clicked on the action it had already filled up. Sampling and hosted party actions are extremely popular, and with limited space available, they can fill up very quickly. Brands launch new actions frequently though, so you should be seeing more actions to apply for soon!