Apologies for my late entrance here, I’ve been away and off grid for a few days!
Some great advice already here from everyone, here’s my take on this, including some further points to what’s already been said. @HelenSilby is right - your WHY is incredibly important, however, what’s also important is giving room to not just the official WHY, but remember that every team and individual user will potentially have a different WHY. You need to try and tap into what that is. You need to be able to prove to everyone the WIIFM (What’s In It For Me) - what specific value will they each get from being an engaged member of the community.
I also agree with @HAWK in that broadcast communication about the launch and purpose doesn’t always suit everyone. There’s definitely a place for this and I wouldn’t not doing, but as the person leading the charge here, get yourself out there. Get some workshops scheduled in with the teams to help establish their WHY and WIIFM. Run webinar-style sessions for people to come in and do similar things, helping them to navigate and use the features.
Also, something that I’ve seen mentioned many times on this very site, don’t be afraid to start small. In fact just look to start small, afraid or not. find who your most likely core members might be be, across the business, get them in there and sharing and engaging as soon as possible. then as you workshop around the business you can steadily add more advocates from those groups of people and teams who are starting to “get it”!
Community itself isn’t the problem often, I find people will happily engage if they understand the WHY/WIIFM.
At my current Community I still don’t have a huge internal presence on our Customer Community, but that doesn’t worry me as the core members that ARE there are doing a great job and will pull others from around the business in as and when. We still have pockets of the business who are coming to me, and vice versa, and we’re discovering new WHYs a year in. I don’t want that to ever stop. New WHYs means more people engaging.