At the conclusion of your session when you select 'Finish', you'll notice the Post Meditation screen. By scrolling through this screen you'll see other users who meditated at the same time as you. Tap on individual user profiles to send a Thanks for Meditating message. Swipe right to browse through more profiles and thank users.
Some users may have disabled this feature, which means the option won't appear on their profile. To check you have these settings turned on: Select Profile > Settings > Thanks For Meditating messages.