Working together on Microsoft 365 files
Microsoft 365 makes co-authoring (editing the same file together with others at the same time) documents easy. Word, PowerPoint, and Excel through Microsoft 365 all support co-authoring files with their respective file types (docx, pptx, and xlsx) as long as those files are saved to OneDrive or SharePoint.
Sharing documents for co-authoring
Anyone that you will be working together with on a file will need to be granted Edit access to the document. If the file you're working on is saved in your OneDrive, then you will need to manage access to the file yourself. If you're working on a file that's saved to an administrative SharePoint site that you've been granted access to, your co-author may already have been granted access to the file. You can check who a file has been shared with following the same sharing steps.
Co-authoring guides for specific Office applications