Multi-Invoice Order Payment

I'm getting a message about "multiple orders" on one invoice?
How can I pay for all of the people in my group with one payment.o

If you don't understand groups, it may be best to start with this answer.

Once your group is set up, the group coordinator will be able to perform multiple actions for that group including payments.  To pay for your entire group, just click the payments button.

Multiple Orders Message:

If you select payments and get a warning message that you must choose an invoice, this means that the members of your group are listed on different invoice (or order) numbers.  As of July 2013, EventCORE doesn't allow for end-users to pay for multiple orders at one time, so you must choose and pay for each one separately.  An administrator does have this option using "Multi-Person Payments" option. So if it is critical that an end-user pay for multiple invoices at once, they should work with a site administrator to get it done.

How do multiple invoice numbers get created? 
The most likely cause is that people make their own individual registration. So initially all of these people are not part of the same group.  If they are later grouped but have their own order number, that must be maintained.  So they're grouped together but with different order numbers which does cause a payment restrition.

How do People Register Individually but Pay as a Group?
There is an "official" way to do this as explained here.

How do we send an Invoice for Payment to someone?
If you have a group of two or more attendees that want to all pay by a single payment check but need to have an invoice there are two ways of doing it.

Simple way: If you can get all of the attendees to be part of the same family or peer group with the same infoice number, then they simply can use the option of "Pay" and print up their own invoice using the "check" option.

Admin way: An admin can use the "multi-invoice payments" method. Basically the administrator pulls up the group of individuals to pay (as explained in the previous paragraph) but instead of actually processing a payment for the group, they would simply hit the print option to print the page. Save it as a .pdf or print and mail it to the person for payment. It will not automatically include instructions for sending a check so you will need to also include how to pay by check.