Adding and inviting friends allows you to build your followers and stay active with people you know (a bit of friendly competition isn’t a bad thing).
You can click on the "Social" icon to find friends. Once in the "Find and Invite Friends" screen you can add friends via your phone's address book, Facebook Connect, or through sharing your unique referral link via social media, email, or text message. You will get 5 Sweatcoins as a reward for each friend that joins through your link.
If your friends are already on Sweatcoin, another way to do this is sending them some Sweatcoins if you have their username. Be careful that you're sending them to the correct user - the best approach is to send a small amount first. Once you have confirmed you've got the right person, tap on their username after you've sent them sweatcoins, which’ll allow you to follow that friend going forward.