Hello Jade,
You can assign a project to each item in an invoice. This is a toggle feature called 'project invoice items' and is located under the invoicing and expenses heading in Settings > Organisation > Toggle Features. A box will appear under the VAT rate box allowing you to choose a project when you go to create/edit an invoice/bill.
If there is one item in the invoice and you're looking to assign multiple projects to it then I'm afraid there isn't currently a feature that allows you to do this. If this is the type of feature you're looking for then you can add this to the ideas sections of the community page and our developers will look into getting it implemented for you based on its popularity.
Let me know if I can be of any further assistance.
Kind regards,
Michael