@Hannahlynne as far as I know there is not an index of users accessible to our access privileges.
I think you can just use the search bar (top right corner) and type in a string of alphanumeric characters then tap enter.
You might be able to make use of the advanced search feature (a little gear icon in the search box.) Just fill in the requisite information.
Once you find someone's profile you can tap on in and the simply tap the follow button.
Edit: you can also search for users in the comment box by typing an @ and then entering the letters. They've programmed it to autopopulate a pop-up box with matching usernames. Then click preview without actually posting the the reply. You can single-tap name (or mouse over) and it'll show an option to follow them.