Share User Licenses among multiple users

If have 1000 users.My org has 25 user license. How to share license between users without purchasing extra licenses?

How can i solve the above issue.I have to buy 1000 licenses for 1000 users.Its not good bcoz some users login in salesforce monthly once or twice to access data. Is there any tool or scenario to implement? In my recnt Interviw asked this scenario. There is a solution for that but Interviwer not revealed answer.