Please go to Walkr > Menu > Social. If you have already explored 20 planets (if you're still in the process of researching the 20th planet, you still only have explored 19 planets), you can then go see the Labs created by other captains and choose one to join!
You're free to join any of the Labs that have already been created!
Or, if you would like you can create your own Lab and invite your friends to join and encourage each other to keep researching new planets!