We currently only log admin users logins, however that would be pretty easy to add in. The easiest way would probably be a very small mod to the signin logic.
In signin.aspx.cs (for 8.x and before) or /controls/signin.ascx.cs (for 9.x), look for the Security.LogEvent reference:
Code:
Security.LogEvent("Store Login", "", ThisCustomer.CustomerID, ThisCustomer.CustomerID, ThisCustomer.ThisCustomerSession.SessionID);
(9.x code)
That logic is wrapped in an 'if' statement so it only fires if the account signing in is an admin user. If you remove that check, it'll record all logins. You could then run a report on the SecurityLog table in the DB to get that info. Note that if you go this route, the Security Log in the admin site's going to show more than just admin logins as well.