Before We Leave

As with most of the achievements in this game, this is not difficult but grindy.

First things first, you create Peeps in Schools, which need to be researched but that can be done very early on. In addition, you'll need to build huts, houses or apartments to house them (with the exception of the Getting Busy scenario).

Obviously, play through the majority of the game to make some progress on this. I played through 1.5 times (I couldn't colonise the inner planet on my first playthrough - for some reason there was nowhere for my spaceship to land ), did all 4 scenarios and a bit of random idling for some of the other grindy achievements. I think that put me at around 3500-4000 Peeps, though there is no way to check.

What I did to grind this out was to expand on Lordacris's solution for Survivalist Plus, essentially save-scumming. I went back to an older save where things were up and running and I had plenty of resources being generated - and where a Whale Attack was about to happen (for the linked solution). I immediately paused the game and placed as many Huts as I could and saved again before resuming.

Every time I reloaded the save, I would get +1 Whale Attack then let the game run for a little bit. As the Huts were built, I made progress towards Property Tycoon (build 1000 Huts) and as they filled up with Peeps I made progress towards this achievement. I also upgraded the Huts to Houses (I'm not sure if building Houses straight up counts towards the other achievement?) for even more Peeps. Remember you can build multiple Schools to speed up Peep production if needed.

On the save I was using my other planets weren't very developed so I just did this for the 3 islands on Planet 1. The more you set up when the game is paused, the faster this will go. If you want to be really efficient, you could expand across several planets and even set things up for some of the other grindy achievements too.

I think this is the fastest way to go about it. It took me about an hour of doing this to get the achievement, but I was messing around a bit and wasn't very efficient. Obviously, ymmv. If you have any suggestions, please comment below. Good grindy luck, Peeps!