Sean - I always ask the following two questions first: "who told you they would pay money for this?" - which is a good place to start to figure out the types of companies interested in your offering. And then "why would they pay money for this?" - which helps flesh out the value perception of your offering for those types of companies.
The thing is, if you can't accurately answer those questions, you should go back and try to. That might mean seeking out potential customers and talking to them. There is a whole movement around this - Customer Development - which is all about product development that is aligned with customer needs. Turns out it applies to pricing, too.
You might find that you will have different types of companies with different value perceptions. This is where Market Segmentation will come into play and you could - as some of the other comments said - end up with pricing plans for different segments that are themselves quite different. Nothing wrong with that. However, as a small startup that might be taking on too much, which means you'll have to make that very difficult decision of whether or not to focus on a single market segment and dominate. Not easy - but nothing worthwhile is, right?
The problem with talking to potential customers is that you can't just ask them what they'd pay! You have to figure out what problem they have, how they want to fix that problem, how you can help them fix it, what value they see in having the problem fixed, how they'll buy (to the point of others here), etc. All of that goes into determining a price for your offering. Remember, Pricing is Marketing.
So what Yammer charges, or how 37Signals priced Basecamp is not relevant here until you fully understand who your customer is, what their value perception of your offering will be, and how & when they'll buy. Using those companies as a reference point is really only useful to see what others are doing out there, but what they are doing may - or may not - work for you.
That said, I Googled you (awkward, I know) to see if I could find what you're working on....ScoopBase is what I found. Based on your tag line "Your team has questions, give them answers" it would seem that there might be a need for some "super users" to answer questions / edit responses and perhaps unlimited "view-only" users. Perhaps you would price based on how many “super users” there are, or maybe based on the other features of your offering. Perhaps some companies – large or small – would rather pay based on certain features and the number of users should be irrelevant. Or not. Those are questions you need to get answered from your potential users. Pricing - no matter what - comes down to the willingness to pay of the customer. The metrics you use to build your pricing must be valuable enough to the customer for them to pay.
Good luck to you on your new venture!