I've made quite a few big projects before, but they would be quicker and easier to make if we could collaborate, or work together, on a single project without sharing a computer, phone, tablet, et cetera. Like if there were a button, where we could pick specific people who can edit that project as well. Maybe it can be made so that you can only allow people to edit if you are following them, and they are following you.

This is rejected.

The Official List of Rejected Suggestions wrote:

2.1 Adding users to work on projects
Having multiple Scratchers work on the same project would be difficult to moderate not only due to technological limitations but also because it could lead to private messaging; for more information, see #7.1 on this list. This suggestion includes live editing (similar to Google Docs) and other forms of private collaboration. If you would like to work on a project with another Scratcher, you can remix each others' projects.

At the moment, a turn-based collaboration system for shared projects is NOT rejected; you can discuss it on this topic.