In order to make changes to an invoice you need to be the owner of said invoice and the invoice must be in an editable status (N, IP, RE, R). You can take ownership by clicking in the 'Owner field' to activate the user list. Start typing in your name and when you find your name select it by clicking on it to change the owner to yourself.
Changing ownership of invoices can be done for multiple invoices at one time. To group change ownership, in the grid view select the invoices you would like to take ownership of by clicking one each invoice while holding your control key.
Then choose 'Actions' and 'Change Ownership'.