If you want to build a BUSINESS that has a "product that we know is very useful for our customers"- To have a business you need customers, customers pay, and paying customers validate that the product is useful to them, otherwise you just don't really know.
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When someone pays you, they are a customer.
Perhaps you have potential customers. How do you know that what you're building will be very useful for them? What will you have that will make them pay you, and not simply use the free service your competitors are offering?
I would say yes, you should charge, from day one. Then you need to find your first true customer. Make them ecstatic about the service they get. Then find another 4.
Then you have a product, customers, and the thin end of the wedge. If you can't do that, how will you ever make money?
Find a customer.
Go and talk to someone you think has the problem you're trying to solve. Find out what they do now. Show them your solution. Get their reaction and build on it.
Sell your product.
This is hard work, but unless you actually sell your solution to someone, anyone, you won't know what your fit is.
Find a potential customer and talk to them.
If the value of your product is directly tied to the total number of users, charge for it some of the time.
LinkedIn, a lot of the value comes from there already being lots of other users when you sign up. Further, a lot of their revenue model is based on selling user attention to advertisers and recruiters. Having users is more important than charging the individual user, although it's still nice when you can convince them to pay up.
Bootcamp, it doesn't matter for crap to the individual user if others are also using it beyond social proof to justify trying it.