PURCHASING > Reports > Supplier information > Supplier portal activity log
(and)
PURCHASING > Settings > Supplier portal > Portal activity log
This provides a list of user/supplier activity in the Supplier Portal (e.g. supplier logged in, changed password, responded to RFQ, etc.).
Alternatively (or in addition), you can receive this information via automatic email notification by enabling the applicable option in the [EMAIL NOTIFICATIONS] Tab of the User Master File.