My team has tried literally everything in sourcetree but we failed. Can someone please tell us what they use to collab in Unity.?Unity collab is an option but we are 4 people so we exceed the limit.

If you have unity student or Pro, u san use the unity collab. You could just create a github repository and make a branch for each of the participants also, it's not the same thing but it helps to organize and let's you update your co-workers codes. That's all I got, hope it helped

Git is fairly easy to use and I've been using it for all kinds software projects including gamejams

There are many free  ways to host your git repository(The central place where your code lives) 

The most popular one is github. Others like gitlab, azure devops, and bitbucket.

Let me know if you need more assistance and we can take some time before the jam to go through it :D