Source control, like git. The most common source control service is Github. Every team member has their own local copy that they work one, and then they'll commit and push their changes to the online master copy. Other team members then periodically pull the changes to their local copy. It's important that you try to avoid working on the exact same bits of code at the same time as that can create a merge conflict (Github isn't sure how to merge the changes back together) which needs to be resolved manually, but in general source control is how all major coding projects manage multiple team members working on multiple features of the same code base at once.