Yusuf You can use the Sandbox environment to test the API by using "https://public-api.sandbox.bunq.com/v1/" as base URL. It's also integrated in the different SDKs that are available should you use them. If you have an Android device, you can use the special Sandbox version of the bunq app to test even more in-depth. It should also work in an Android emulator.
For more info: https://doc.bunq.com and https://beta.doc.bunq.com
Can we open one shared(more than 2 people) bank account to test our product to be used by developers?
While it's not allowed to open more than one bunq account per person, you're of course free to open one real account for each person (as long as they do so out of their own intent) and create Joint accounts as you see fit. You can create up to 25 sub-accounts for free if everyone has at least an Easy Money plan, which includes Joint accounts. So that wouldn't be completely free but still give you the ability to try out a lot of stuff in the production environment.
For development purposes I would mostly use the sandbox environment, though. That's always free and don't have to use real money. You can just send payment requests to email@example.com and get as much money as you want ;)