How can a platform where users would benefit from sharing and connecting with each other, attract initial users?

You need to think about the user experience for the first 50 people, as you are doing.

If you're not solving a problem for them, then there is no value, so the user base will not grow. It's always easy to envision a product or service which if you have e.g 1m users is valuable; but the hard part is getting there.

If your service is ONLY valuable with 1m , or 1k users, then you need to find a different reason to onboard them, so they find some different value until you get critical mass to begin delivering on your vision.

e.g Facebook started by solving the problem of who is in my year at college to flirt with (hence the Poke button, amongst other things), later it delivered alternative value. They also just "onboarded" everyone in one go, which helped. So, you need to narrow your focus to a segment of your target audience you can focus on, target, and onboard, by vertical, geography, or some other measure. Get the first 50 people, or even 5(!) people happy, because you deliver them some value, then you can start building on that start.

My favorite phrase as it relates to this is: Fake it, til you make it!

The question I would ask is, can you be hyper-focused on a specific small group of users, and seed content specifically for them? Facebook started with one specific school, and seeded itself with information on classmates. When people joined, they found it engaging, and kept coming back.

Identify a group in your city and approach them about using your platform to solve a problem they are having. Get them sold on your platform. Get them onboarded. Work with the leader of that group to ensure there is content and activities for those users.

Use that group as your alpha group, and learn from them what works and what doesn't. Validate your platform is a solution they are happy to use.

Then repeat the process with a new group.

It will be a very manual process to start, but that's how you'll get those initial users on your platform. Just keep grinding away, and it will start to grow.

The platform would need more of a value add to get consumers to be early adopters. With sharing being a uniform aspect of the Internet more and more, what you need is a data resource, creative content or interface that sets you apart. In other words, consider what kind of shareable content your brand best associates with, and find suitable online content providers to create partnerships with. This gives you a base, allies and a growth strategy, all at once!

Happy to elaborate more and help you out!

In the beginning, try organizing specific times where everybody will be online simultaneously. In other words, treat the website as a real-time conference, party, or networking event with a beginning and end.

Without scheduling like that, your platform will be a ghost town at first.

Also, consider seeding the group with a small number of guaranteed attendees. Ideally these people will be chosen to add value for the other guests / users. Either they'll be well known enough to act as magnets, drawing other people in to participate; or else they'll simply be interesting enough to elevate the dialogue and maximize engagement once other users appear.

Those "seed" users would need to be specially recruited. Maybe you can guarantee they'll be around without paying them anything; but even if you did have to pay a few featured guests or staff members, that may be worth doing. They solve your chicken-or-the-egg problem. They're the initial clump of snow that allows you to roll a bigger boulder and build a snow man.

Back in the days, was easy to create a platform and have engagement - BUT TODAY - there are hundreds of social platforms but none of them work - and the reason is that technology is evolving - everything is evolving around us - so this means that a Marketer needs to think twice before creating a platform.

At this time, platforms that only talk (WebhostingTalk,stackexchange) will decrease in engagement and majority of users - will go to Platforms that have an application+talk.

Example:
WebHostingTalk - here users only talk and slowly - this page is dying. The same content you find on on WHT - you can find everywhere else - so looking from the ROI prespective, a platform such WHT will not work out.

On the other side, GNS3.com - here is platform based on user activity + application. From this platform, i have two types of engagement (Talk + use my software). By using my software, users will start to give questions, faqs, answers - so this way your platform community will grow.
Examples of this kind of platforms are:
Gns3.com
Spiceworks.com

So in conclusion - for a platform to have engagement,activity,actions - you have to provide them 2+ of engagement. (Talk, download your application). Combining this two - you will have a strong community platform.
Take example from GNS3.com - they are a great example for anyone who want to create a community platform.