External users - how, how much, etc.- please reply quickly, been waiting for support for days :(

Hi,

First off, I did contact support days ago and have had no reply at all so I could do with a quick reply please.

We are working with a marketing agency. How can I add them as external users to our Zoho One apps?

What apps let us do this (we need at least campaigns, crm, social, marketing hub).

Is there a cost to add external users like this?

Please advise.

Thanks