I've helped several businesses do this.
1) Differentiate yourself - make your portal highly compelling for small-to-medium businesses.
2) Give it for free to businesses that are recognizable and will allow you to feature a testimonial from them. This will build credibility. I like to go after a single location in a chain, for example get a single McDonald's manager to use it, a single Chipotle's store, a single Applebee's manager - and now you have a powerful social proof client roster.
3) Do you need a lot of users (people with the phone app or who are using the tech) in order for businesses to benefit? If so, that presents another set of complex challenges. I can help you create a strategy to get to market and revenue fast with this challenge by phone. It's the number 1 reason startups like yours fail - unable to get traction from both users and businesses simultaneously. If you don't need both sides at once, then...
4) Get feedback from the free users, then ask them if they'd be referral sources. Then begin an outreach campaign - you can use techniques from Mixergy or set up a call with me - it will require a custom plan based on the specifics of your business once you reach this point.
Are you trying to create something that already exist? There are a lot of social networked portals out there and to get businesses to sign up, you would really have to demonstrate creditability and make a strong case that these small to medium businesses would benefit from your portal rather than those other networks out there.
Launching any project involves a million and one moving parts. Each of them needs to be chosen with purpose, fitted to the others, tested, greased up, and set in motion.
Really, your question is a million and one questions.
Obviously, you'll want to look at the portal itself in terms of functionality and design.
Obviously, you'll want to to identify and understand your customer base, their needs and their reactions to your service.
Obviously, you'll want to be aware of competitors or existing solutions and position your service and sales pitch in order to be convincingly different / better.
Obviously, you'll need a marketing plan to reach customers -- and a way to measure its success.
Less obviously, perhaps, you'll want to think about your own branding -- including the brand name -- to ensure that all the energy you spend putting your product in front of folks results in a positive and lasting impression. If I can help in that regard, say hi.