When I took over our G+ account, we had 50 followers, and we now have more than 353K. Here are some suggestions (pretty much standard across all social media):
- Start by observing and interacting. Purely pushing content and trying to gain 'numbers' isn't productive, and users don't respond well. Join communities that share your interests, and begin interacting and finding out what people look for.
- Conduct hangouts. If you have a valuable message to share, break it down in an accessible way through a hangout. Here are some great tips from Fraser Cain https://plus.google.com/+FraserCain/posts/PaeeynDx34L
- Create content. Content is king everywhere and the same goes for G+. To quote an old editor friend, "People don't want products, they want solutions." Find out where there's a need, and create the solution.
- Don't just share your own content with links. Despite the grillion articles out there today touting social as a great way to 'build links' it's completely see-through when that's all you're doing. Share other people's stuff. Post fun stuff that doesn't include a link to your content occasionally.
- Trending. Find out what's trending, and how you can fit in naturally. Don't just push a piece of content and slap a trending hashtag, but see where you can fit into the trends.
Hope this helps!
Lisa