For Collaboration, the most important aspect is project planning, task delegation, and management to ensure consistent and steady development.
Foremost, please...
Get Hands-On with Version Control with Your Team Before Your Unity Project Gets More Complex!
At the earlier stage (gryboxing), you and your team need to set aside collaboration time in Unity using Version Control or Github to greybox your environment. Make sure to coordinate and get hands-on with the process!