Ideally, if you're using a single license (standard), accessing and making any changes on the file can only be done one at a time. If you wish to add a user that can make changes while you're working with your file you have to purchase another license for that user.
However, you can add up to 3 users to work on your company file. Thus, any changes made by the other user will not sync to the file with the other one.
Here's how to add a user:
Go to the Company menu, then click Users, and then Set Up Users and Roles.
Add their user name, password, and their roles.
Then, click OK.
I have here a helpful user-guide that can help you: