It was very Easy to join. I see the pool has grown to almost 300PH. Around what level does this need to get before we have a reasonable chance of finding a block? Can you please explain the “promotional bonus”?

At 300, it's about 11 days between blocks on average (likely a bit shorter in practice) Normally a block reward is split across the past 8*difficulty work performed. But that much work hasn't been done yet, so early miners would get a higher payout for a while

What do you mean by "past 8*difficulty) ?

The expected work to find 8 blocks