The Overwatch Team (internally at Blizzard we are called "Team 4") is comprised of about 100 developers at this point. The disciplines who comprise the team are Audio, Art, Engineering, Production, and Design. We also have two full-time Business Operations people and an esports director who are part of the team. Our size fluctuated throughout development from around 40 developers to about 75 at launch. Around launch we brought our audio team onto the team full time (they were technically a shared, central resource although primarily focused on OW). We love that group way too much to share them with anyone! We also brought our automation group onto the team (thus bringing us to our current number). The automation team is particularly awesome -- true unsung heroes. They had literally hundreds of thousands of "players" playing the beta for most of the time that the beta was running. The reason we were able to have such a smooth launch and keep the beta small was because of their fantastic work.
We also have two amazing "embedded" groups that sit with us and we consider them part of the team as well. This is our dedicated Quality Assurance team and our dedicated Community team. Quality Assurance makes sure everything works and is stable. They call us out on our bugs and try to get the game as rock-solid for you guys as possible. The Community team does all of our community outreach activities from our social media channels (Twitter, Facebook, Instagram etc...) to influencer interaction to scheduling stuff like developer presence on streams, our "Developer Update" series and generally, telling me to capitalize and punctuate my sentences.
So that's the "immediate" team that works on Overwatch. But we're just a small part of a bigger picture. We get amazing support from so many groups. Blizzard Animation makes those AMAZING animated shorts you've been enjoying. They are part of a larger organization called Story and Franchise Development that does so many great things like the Overwatch Comics, the awesome collectible statues (like 76 and Tracer), our gameplay videos, our training videos, our developer updates... etc.
We have a licensing group who makes all of the cool products that you can get on gear.blizzard.com (wow, shameless plug!). We are lucky to have an entire group in Battle.net that maintains the launcher, keeps our games running, allows for cross game chat, hosts the web and mobile teams, helps make voice chat a reality, allows for cross-region persistence and the list goes on... We have an amazing team of lawyers who makes sure that all of our "real world" representations are fair and legal (for example the Hollywood sign in the Hollywood map). They do amazing, unspoken work to keep our game safe from cheaters as well. Which brings me to our anti-hack, anti-cheat and security group. This team is constantly hard at work and yet we can't really discuss their success publicly at all.
We have a centralized esport group that helps organize our esport activities and interacts with groups like ESL. We have the world's most amazing marketing team who did cool stuff like the Colossal Collectibles around launch. We have a whole IT/Networking/Live Ops organization that makes sure Blizzard servers are running worldwide at all times. We have a public relations team that interacts with the press and schedules any interview you see with a OW team member. We have a BRILLIANT business intelligence group who provides us with statistics and analysis. We also have a great executive team running the company who help guide the high level strategy of Overwatch. Many of these guys like Mike and Frank (CEO and Chief Development Officer) spent many years as game developers... so they really understand what we do.
There are even more groups here at Blizzard that are helping us all the time. We have the world's best Customer Support team known to existence (fact). A great HR team... I know I am leaving out some well-deserving-of-praise folks -- and I apologize. But you can see, we get a lot of support.
But at it's core, it's about 100 gals and guys trying to make cool stuff that makes you guys happy.