I did not participate in Meta-Events Rush during tier 1, but I did one event during tier 2 - it was a few hours ago. I've just logged in again, tier 2 is still going but I've just got the chest with reward for tier 1.
Conclusion: participate now and you should still get all previous rewards as well. :)
External link →Quick clarification. The way the cross-map data works, you have to log in to one of the maps where the community goal bars are displayed to get the community goal rewards. That would be all of the maps where the tracked events occur, or any city. And yes, once you've contributed at all you will be eligible for all community goal rewards.
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