We are a few developers, working on the same project.

I have an application still in sandbox, thus, random users are unable to access it.

The problem is that my co-developers cannot access the application either. Even though I have added them into "Manage developers" part of "My Apps", they still see "Sorry, the app is currently private. You must be a friend of the app to view this page." when watching the app and "You are unauthorised" when they click "Add this app" in the app profile.

Is this behavious normal and why? What can I do to make my co-developers able to see the applications?

They all must also be approved developers.

Have they all submited and recieved developer status confirmation?

 Here's what I did to resolve the issue in my case. 

Log in as the application.  When you created the application you specified an email and a password - use those. 

Onced logged in as the application send a friend request to the user or users you wish to be able to add your application to their profile.

Once the users have accepted the friend request they will be able to add the application.