Quite similar to what I had in my last Shadowrun campaign.
I cut the interaction into different blocks.
The Norm
1. Show the Normal and make the PCs like and love and interact with NPCs of a specific turf. Standard jobs for these people who are the poor but have high dreams of becoming something. E.g. there was a low-on-luck song writer.
2. Introduce new development projects "for the poor". Located in danger/hazard sites. Greenery and work and food and sheltering in an area that was believed to be polluted, but which was gradually restored. Or so they say. That seemed like a great idea and something that was contrary to the expectations of a big company. This time, it seemed they were not in for the profit of the rich. The PCs got to help on the side lines.
3. Then, new products on the market. People were fed, got a home in these new centers. People came away from the streets.
Suspicions
4. There was already a hint of people loosing contact with those in the program. At first, it was believed that they didn't want to keep contact with their old live, including their contacts. And other people became a little strange. Contact of theirs. Nothing to much, though.
5. Then one person, very worried, asked the PCs for help. It was very unusual that a specific person who entered the "New Life" program didn't reply at all. But the PCs weren't able to make contact also. The got to know the friendly face of the programs though.
6. Hobos from the streets who had antagonized the "New Life" program just vanished. Simultaneously, media showed the CEOs of "New Life" and how good their project would work and help the people.
Danger
7. And then, one of the people of the program flees. He calls. The PCs are tasked with protecting him. They find him in the middle of an abduction. The other side was fast also. Fighting the abductors, the PCs suddenly realize, that they were no humans in those hazmet suits and under those helmets.
In my case, they were ant insect spirits inhabiting human hosts. There were smaller workers and extremely strong fighters. But there were also those spies that still held a lot of similiarity to humans and lived in the same society.
And they made good use of the workers. One of the interesting parts was the additional food that sub-companies of New Life brought to the market... using human components and addicting substances that made the hosts less resistant to the hosting.
For a hive mind, I used several tricks at once. They were productive. Very. And they had several simultaneous plans. Getting grips on the power players of the city, ensuring their livestock, their required materials (for machinery and food) and they took it with force and a great number of bodies, while not showing who took it. They regularly needed to rent a massive amount of cars (which eventually led to them).
The enemy of the hive mind is never the individual, as they can be dealt with or taken over, but to keep out of sight until they are ready. Until they could swarm the whole city from below (sewers, tunnels, abandoned buildings). And when that happens, they would already have been in place to have taken over important people of society (police chief, politicians, media, etc.).
E.g. I started it all off with "an accident" at an important food production company. Nothing too wild. But in the next months food prices increased and the summer heat swept in. Drinking water was stable in better town areas, but the poor couldn't possibly drink from the faucets, so drinking water got scarce in the shops. Repeated reports of burglary. The situation was bad for the poor. That was when the new cheap products hit the market and seemed to solve the problem...
So, to sum it up, for me it all boiled down to presenting the Norm and have the NPCs grow on the players and then slowly introducing change. And as soon as they pick up on what is happening, they work against the clock, knowing how bad it will turn out to be. They were in the middle and I used the NPCs as triggers. Horror as for the tone.
(I have several more detailed descriptions of this in the forum. Just hit me up for links)
