Hi Jonas,
Option 1: Every one of you registers a trial account. In this case you share the sources in source control system of your choice (e.g. GitHub). Then you make changes locally and deploy and test on your trail account.
Option 2: You share the user and the trial account. I think this will be inconvinient, because you have only 1 running application limit. You will either override others changes or will have to wait until they finished their work with the running application.
Going to customer/partner account should be considered, when you have developed your application and want to run it productively.
HTH,
Alex