There are multiple methods for doing this and no way to tell what method was used with your database from the information you have posted so far. You'll either have to learn enough filemaker to investigate this yourself or hire a consultant to take a look at it.
To investigate it further:
Select a layout where you can see the invoice number.
Switch to layout mode so you can see it's field and table names. You may have to double-click the field to see all of this.
Open Manage | Database | Fields, find the table, find the field and see what type of field it is. Double click the field in this dialog and check to see if it uses Auto-Enter by calculation to insert a serial number.
You may have a button that starts a new invoice. In layout mode, double click this field to see the name of the script that is performed to start the invoice. The script may be assigning your invoice numbers. Use Scripts | Script Manager to find and open the script to see what it does.
All of the above will require you to learn more about Filemaker and/or post descriptions of what you find here or on another filemaker forum.
I found the script... It is an If function.
If[Get(UserName) ≠ "Name"
Can I put more than one name in this if statement?
Thanks for your help!!!
User name is a preferences setting. Since just one computer is not working as you want, select Edit | preferences... and see what name is entered in the User name box in this preferences setting. You can simply enter the user name that works for the other computer.
If you want to enable two user names, you can modify the expression inside the If step like this:
If [ Get ( UserName ) = "Name1" or Get ( UserName ) = "Name2"]
Note that these must be the two names that aren't the "name" currently in the script in order for this to work.