There are two ways you can send a friend request. If you’re using Search, you will receive a list of Accounts relevant to your search terms. Swipe left on the Account you want to befriend, and touch “Add” to send a friend request.
The other way to send a friend request is by going to an Account Wall. Touch the sub-account “options” profile badge icon to go to the Account options screen. Touch “Add friend” to send a friend request.