As a google apps administrator, How to monitor the list of users who are using google apps email (say over the weekend)
You could try logging into Google Apps and checking the Reports tab.
Details on how your users access Email can be checked by clicking on
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The email clients report explains how users in your domain access their hosted accounts on a day-by-day basis. For each day, the report lists the total number of accounts in your domain as well as the number and percentage of users who accessed their accounts using WebMail. This report does not include suspended accounts in the account total.