Ensure your profile was compelling. Build a business page.

Join related Google groups. Contribute to those conversations.

Follow influencers in the space (sometimes they follow back, though the follow first strategy works better on Twitter).

Write blog posts and share those on Google+ (and other social networks).

Use other social networks and tell those followers you are also on Google+.

Ensure your web site has prominent icons showing that you are on Google+ (and other networks).

Ensure your Google Authorship is setup on your Google+ page and your blog.

Look for other blogs in your industry that are open to guest blogging and post articles on there as well. Ensure your Google+ Authorship is linked to those. Ensure your author bio/profile on those other sites (or footers in the articles) link back to your Google+ page.

Post previews on YouTube (if your courses have video material). Share those on Google+. Link your YouTube channel. Links in YouTube video descriptions to e-mail sign up landing pages.

Build custom landing pages for various circumstances.

Of course you're going to want to then push people to your site for the e-mail opt-ins. So keep tasteful notices on your blog/site so when people read your content they can sign up for your newsletters. I personally am not a fan of popups in people's faces. Tasteful banners and fixed position panels that tuck away at some edge of the screen are great.

Give away resources in exchange for e-mails. Have some free PDF that would be useful and meaningful to people? Maybe it follows up on a blog post even. Provide a page that requires people to fill out your e-mail list form in order to download the document.

In your case you might want to give away a portion of a course or something. Maybe your e-mail list highlights other courses. So now you just gave someone something of value and are leading them to more value (that they should be happy to pay for that next time around).

Twitter has lead generation campaigns that work quite well and integrate with MailChimp. I know you said Google+, but you should seriously look at multiple networks.

Then I'd look into ads on social networks. Extremely targeted. Small budget. This works well for optins and as a byproduct you get your name out there and do get followers from those types of campaigns as well (even though what you get charged for is leads and the goal is leads - it still brings you some followers).

To sum all those steps up: You need to generate content that people enjoy so they are more interested in signing up for your e-mail mailings. People want a reason. You use Google+ (and other networks) to reach out to people beyond your normal web site traffic. You get into circles and communities within these networks (and other sites) as well. Though don't blatantly spam.

You mention the company had developed free content assets to 'bait the hook' for optins already. So whether it's using the most valuable piece or pieces of already-created content or ruining a contest to give away one or several premium pieces of content or free registrations/downloads to the course; create a squeeze page for option/sign ups with either immediate download of that content or registration into the contest/giveaway.

A couple of optin apps I have experience and success using are option monster and pop up domination.

Both work as plugins for WordPress and pop up domination can also be installed standalone to work on any website/page via embedding a simple longer of generated javascript.

A great way to use either app is an "inline" form on your page along with graphics/copy and an exit intent pop up that prompts a user as they attempt to leave the page.

Good luck!

The simplest method of growing this would be to ensure you are putting out relevant content showcasing your company across the largest applicable groups not only on G+ but also sharing the content you post on G+ to prominent groups across LinkedIn and Twitter as well. By combining the business focus of those 3 networks in tandem with solid content your corporation can quickly become well established in the first 3 months.