Enable selective exchange mailboxes for archiving

Hello

We are new to EV for Exchange and  had our  EV environment set up by a Consultant, who at the time set up our Mailboxes for IT Dept staff only.

We are now ready to gradually introduce all the Exchange Mailboxes (400) however  we want to add a few at a time, e.g. Finance Team, HR Team, Directors.

Is there an article somebody can point me to as a guide

Thanks

Lois