How to set multi-user mode for my company file, so multiple users can make transactions simultaneously ?

Welcome to the Community, @pat182. I can share information about multi-user access in QuickBooks Online (QBO).

QBO allows you to add multiple users to access your account and work simultaneously. 

Here's how:

1. Sign in to your QBO account as a master admin.
2. Go to the Gear icon, then click on Manage users.
3. Tap on Add user.
4. Select a user type. Then select Next.
5. Choose the access rights for the user, then Next.
6. Select the user settings, if applicable. Then select Next.
7. Enter the name and email address. Then select Save.

You can also check out this article for more information: 

• Add and manage users in QuickBooks Online
• (Multiple users) Save time and work better together

In case you need to add an accountant to access your file, use this link as your reference: Invite an accountant to review your books in QuickBooks Online.

If you're using QuickBooks Desktop (QBDT), check out these articles as your guide about multi-user access rights:

• Features available and tasks you can perform in single or multi-user mode
• Install QuickBooks Desktop on a server

If you have follow-up questions about multi-user mode, let me know by adding a comment below. I'll be right here to help. Have a good one!