Click the Search tab then  "Advanced Search"

To specify a  date of a date range , click here.

You only want memberships that are completed. Click "Contribution Status"

Next to limit the results to memberships click here.

You will get an alphabetical list of the folks that joined in that time period. 

Once you have the list of membership you want to process, you can perform any number of tasks like merge and print a membership welcome letter, add records of activities etc. 

Jorba Membership Fulfillment Workflow Lookup Memberships