This might not help as I'm on an android... but I went into Instigram and the 5 options on the bottom (home, star, camera, heart, and then what looks like a badge) and clicked on the badge. In the top corner on the right is the search magnifying glass and I clicked that. Then it lets me search for a user... I type in the username I want and then hit the "follow" button. Voila!
Open Instagram, look at the bottom row of icons. Click the one on the right (kinda looks like a badge). It should bring up a list of menu items, and the first should be 'Find friends.' The third is 'Search Instagram' where you can search for a particular username if you want. Once you find someone, there should be a button to click that says 'Follow.' Click and voila! You're done.
I've added a bunch of you here,my username is neonlitdaydream.So glad this came out on android!I love it,finally have a place to share my zillions of phone pics!