There are lots of factors and ways to advertise for beta testers, have you thought about the following questions:
- Do you have a landing page?
- Do you have the option to join the mailing list if you don't have an invite?
- Does your landing page have a clear call to action?
- Does your landing page have a clear explanation as to what your platform is?
- Do you have clear concise pricing, or a clear message if its free?
- Have you reduced to minimum the number of steps to signup?
- Do you have analytics in place to know about your visitors?
- Are you tailoring the landing page message by referral source?
- Do you offer an easy way to contact you? Are you engaging with signups after they sign?
- Do you have a list of top influencers in your field and are you currently stalking (um engaging) them across various social networks?
- Do you have a social media presence across the networks for your platform?
- Are you providing simple mechanism for signups to shout about you?
- Can you explain your product in about 100 characters?
- Have you reached out via email to top influencers in your field, offering access?
- Do you have a demographic or interest groups you can target, ideally via the top influencers you are engaging with?
- Can you advertise on your demographic and interest target groups?
Do you engage with people who contact you, do you follow up if they haven't signed up?
What is the average cost per acquisition of new sign ups?
- What is the maximum you can afford in terms of acquisition?
- Are you currently using AdWord/Facebook Ads etc?
- Are you retargeting visitors who haven't signed up with display adverts?
- Are you using promoted stumbles?
It's very easy to spend a lot of money and not get results if your onpage is not spot on, if paid for advertising is not working then something is wrong.
- Have you looked at your competitors (or similar but different niche) advertising strategy?
Laying the ground work for signups to beta is on the whole the same as laying the ground work for any site or project. Identify who you want to sign up, target influencers both individuals and other sites in your industry, build links and generate discussion (surrounding but doesn't need to centre on your platform), keep up the engagement and follow up anyone who is interested. Retargeting is a cheap but effective way to re-engage people for example. Use social media to drive home the message and make it easy for your signups to spread the message for you to the point that it might be worth incentivising tweets and Facebook likes.
Obviously this all comes at a cost, you need to identify how much each sign up is costing, how much you can afford if either of those is zero you have done your math wrong. I suspect while there is more effort in targeting interest sites then say craigslist and other bottom feeders you will find the reward substantially higher.