I founded and admin the largest Pathfinder RPG community on Plus (3,300 members and rising). We have consistent interaction on a constant basis and very little spam. I started it in the first few hours after communities became an option.
I would offer two things:
* Engagement is often dependent on topic. With any web endeavor it is important to meet your audience on their own ground. Like every other platform Google Plus will better for some things and worse for others. This is where a bit of data mining can be key.
*. Providing valuable content is vital, especially as more and more weight is being given to social signals in search. I know it might sound trite but contributing great content, and by extension actively seeking out and inviting others who do so on the same topic, is essential.
Once you get a Plus community revved up it can be an engagement monster.
On a side note, an interesting experiment is going on in the tabletop gaming field - +Kobold Press
has shut down their highly active forums and transitioned to a Plus community, I'm curious to see how things work over the long term.