I'm not sure what happened in the 1st round, but as for the others, what it means to say is that you cannot cheer if you fought in any fight at all. The reason being is that since you fought, you will receive the group coins as the prize for participating (plus packs later on if your group leader gives you one). Those who did not fight at all will not, and so cheering is a way for them to get at least a little bit of that.