Multiple contacts to client

Hiya... me again - sorry!
I was just wondering if it was possible to have multiple contact names against a client - an example:

I have a client who is an advertising agency and I do web development work for a number of different accounts of theirs.

I'm adding each account/website as a new project, but depending on the project the invoices need to be addressed to different account managers which I can't seem to do at present. A field against each project for Project Contact or something would mean my invoices get addressed properly.

Thanks