1. Canva

The non-designer’s design tool

Try Canva here (Create a free account)

For anyone wanting to design anything, on your own or with others. No design experience required, and it's free forever.

Team approach

You can invite people to collaborate - BUT better to create individually

Here's a how to get started after you get logged into Canva 
How to Use Canva: A Beginner’s Guide

There are lots of ways to share Canva projects. Or add collborators.
Click on Share button in upper right to reveal options