There are lots of different forms of credit, and so it will depend on the relationship between you and your friends.
If it is just moral credit, you could easily add a credits page in your app, or put information about them in the app description.
However, often these things come down to ownership. Conceptually the developer account an app is published under represents the entity (company / organization / person) who owns the app. If you want it to be reflected that you all own the app, I'd recommend:
- create a new gmail address and a new developer account for things you do together called whatever you want to call the organization: eg "Me and My friends software".
- then in the Google Play console add all of you as users with full permissions
That way it is very clear this app belongs to all of you. It also makes things easier if you want to found a legal company etc later.