How to manually start archiving for multiple users?

Hi, 

I have a bunch of users ( around 500) already enabled for archiving, but due to their huge mailbox size, they have a lot of unarchived mails. we are planning to set limitation for their mailboxes, but before doing that we need to manually archive their emails to reduce the size of their mailbox, then they wont face any issue for send and receive. So how can I select only these users ( they are already added to a provisioned group) and start their mailbox archiving. Or how can I start archiving for this provisioning group.

EV Version 10

please help. 

Thanks.

Nazir