How can I mark a party as a Supplier?

So I have marked one of the products as “purchasable” and want to add a supplier, which I have added as a party — but the “+” button in the supplier tab is greyed out. I guess I must do something in the party setting, or maybe activate a module, but I am not sure what to do here.

You must be in the “Purchase” group to setup suppliers on product.

Thanks, that was indeed what I forgot about!