You can "favorite" a merchant by tapping on an offer, and in the upper right corner, locate the heart icon.
Tap the icon, and it will turn purple and a message will show that you've added the Merchant to your Favorites.
You can see your favorites by tapping on the three bars in the top left of the main screen, then selecting My Favorites.
This will let you easily see if there are offers available for your favorite merchants. You can remove merchants from here, as well as within the offer, by tapping on the heart.