Hey there. I took a look at your landing page and it's an interesting premise. We are also in alpha stage right now for Saleswhale (
http://saleswhale.io) since 4 months ago, and I've learnt alot about the process.
One thing I've learnt is to look at the pre-public phase (alpha, beta) as a continuum rather than as a hard milestone. The whole purpose of this phase is validated learning, and to make something that people love. Growth is generally not important to us now, although we are still tracking lead velocity (number of inbound email signups weekly).
Not pursuing more users right now at the expense of product development and customer development is generally the smart move, provided your initial set of alpha users are well qualified, and are the right target market for your offering. Reason is that you really don't want too much noise to signal ratio, and too many unqualified users will cause your product to be dragged into different directions or trying to be everything to everyone.
Just to go into specifics, we currently have over 800 alpha signups, and we are rolling out by cohorts weekly to different groups of alpha users, which we are prioritising by a weighted heuristic on how long ago they signed up and potential fit based on their industry and role for our software.
We are tracking engagement aggressively - every single click, action and event gets pulled into our analytics database, and we have a single most important metric that we are tracking - number of interactions a day per user. You may have your own metric that you are tracking. Until we see this metric improve exponentially month on month, we will continue to stay in alpha/beta to work on improving the product and features rather than launch to the public. Because what's the point of getting so many users, if they won't engage and you can't keep them?
Hope that helps, and all the best for your startup!