There are definitely some advantages to growing slowly, but I suspect that the growth of this service would still take significant time even if you were to aggressively promote Groups.io. At the moment, a significant number of people who would be willing to join public groups sign up, click find groups, discover that there is nothing there and then leave and then never come back. Or perhaps they decide to come back in a month or two and if nothing is still there, they are likely to give up or forget about it. So there is a significant opportunity cost of the current situation.
I wonder if the existence of seeded groups would discourage people forming groups more than a lack of users on the service. I think the priorities of people starting a new public group are firstly, is there anyone on this site who might join my group, secondly, is this a good platform to host my group on and, much further down the list, is there a "similar" official group. I mean, the existence of an official Web Development group is likely to make it easier for someone create a jQuery group because it brings a bunch of web developers together into a space where they can talk to each other and agree that jQuery discussion has grown to the point that it requires its own group. Secondly, if someone has their own vision of how they would run a web development group, why not join as a member and apply to become the moderator once the group takes off? If they are later selected/elected as a moderator, then they have had their audience built for them.
Alternatively, if someone is intending to bring a pre-existing group, then they are likely to be less concerned about competition than about how easy it is to migrate over and how Groups.io features better meet their needs. If the group has a strong community, then they shouldn't be threatened by official involvement in a very few seed groups that ends as soon as the group reaches critical mass.
In order to get this community started, only 4 or 5 groups would need to be seeded. These groups would only be featured so long as there weren't any general interest organic groups with higher amounts of engagement. I suspect that these groups could gain a minimal userbase from just being featured for a few weeks. There wouldn't need to be a flashing banner proclaiming that these groups are non-organic, just a post in the group itself explaining that the group was officially created and how it is going to be run/transition over to community ownership. As soon as these groups are passed over to the community, the whole issue of discouragement ceases to be an issue. No-one will care that a group was at some stage official. The only thing that would cause people to feel threatened would be if you planned to keep control of these groups because then you would have an incentive to use the advantages of the platform to ensure that the official group succeeded and they would be worried that you might try to compete with their group next. Furthermore, people coming on board after these official groups exist will take their existence for granted. No-one complained about StackOverflow or Superuser or Server Fault when StackExchange opened up their platform (but you can be pretty sure that whoever founded Apple StackExchange would have if SuperUser had been created after).
Anyway, it may be worth noting that the idea of featured groups and the idea of official groups are two separate ideas. I would suggest that featured should be manually curated at the start to avoid groups like Poop showing up and to allow you to prioritise groups that appeal to a broader audience. Alternatively, "Featured" could even be a formula that took into account number of users, number of posts and last post. If you need help coming up with an algorithm, my field of study in university was mathematics, so I could create a formula to match your requirements.