The first question I would ask back to you is how many clients you have currently. What is your retention rate? Your capture rate? From there you could develop a strategy to capture the next 5,000. If you haven't enlisted certain marketing channels or networks, you may be able to make a prediction about how many add'l clients that will bring you. Hope that helps!
Great question, with a million different ways to get it done, but none without a lot advertising/word of mouth in some way, shape or form. I remember at one of my startups, we grew 350k+ subscribers in a couple of months, but it was very painful and expensive. Today, you're in luck. There is a new tool you might try instead - Kickstarter. Post a campaign on Kickstarter: raise some much needed capital without giving away equity, sign up some very interested people to the next version of your productivity tool and get immediate market feedback. If you run the campaign for 30 days and get more than 5000 people, which will all be highly motivated evangelists, to sign up...you're a double winner. You will have met your goal and gotten additional funding. If you don't, it's great marketing feedback that something is not quite right yet with the product or the value proposition. I would be happy to provide additional thoughts on how to do this with a follow on call.
Take a look at this article about DropBox.
The Company was able to have a 3900% growth rate over 15 months. This outstanding growth was created by implementing a successful referral campaign. If you can create a referral campaign that gives your users something in exchange for their efforts they will feel more compelled to invite people onto your platform or service. This type of growth campaign can be difficult to implement, but if you are at a stage prepared for growth, then you should be able to seamlessly integrate new subscribers onto your platform without too many (if any) hiccups.
Before you begin a referral campaign, remember that customer acquisition can be expensive. If you're only motivation behind the growth in customers is financial return, then consider whether your pricing structure is optimal before you rush into a referral campaign. You may be able to substantially increase your revenue by increasing your prices or looking for creative ways to reduce your overhead. Either way, when you are ready for rapid growth on a subscription-based service, (freemium or not), referral campaigns can be one of the most cost-effective ways to grow your customer base.