Mass members is not necessary immediately but an active base is. New forums have a big problem -- there are only a finite number of people posting at any time thus there are few topics to post about and at the same time, people who come and who may join the site aren't impressed because of the lack of activity. Without a "critical mass" you won't create enough of a base of members to keep the content flowing and the discussions happening. This means visitors don't register as often, members don't return as much and basically your forum goes no where fast. I've seen some communities get off to a great start but fail to lose their critical mass and spiral down to nothing.

In terms of an opening marketing campaign, you should be doing a bit of everything. Some methods have a shorter payout than others but there is no one magic formula. You're going to want good SEO as it's free and can be a huge traffic source. Local/ offline advertising is great as it lets you reach into people's lives when they aren't considering a community. Adwords and banner advertising lets you drive in targeted traffic on demand (based on your budget) without the wait. Link exchanges and "viral promotion" can bring in more members without cost as well. All of these techniques work best when combined... after all, your SEO rankings can drop without warning, your AdWords campaign can run out of money, your link exchanges can be removed and so forth. More importantly, it's well known that to create any brand awareness you need multiple impressions of your name... so you want to hit people online, offline, on google, on other sites, anywhere you can to get them to consider checking you out.

Good luck!

Hey Ted, do you have a spreadsheet template that you use to track KPIs that you could share? :)