Easy to find out when your Members have logged into the system by running a Member Report.
Settings - Reports- Members
You can filter which Membership Type or do all. Scroll down and enter the date you want to begin with. So if you imported your members and sent out your Welcome Email on December 15th maybe use a December 1 date
Determine what information you want the report to give you.
Click Run Report. You can also save this as a filter to use later.
Based on your perimeters it will give the data that you can export to excel or a pdf.