To send a Friend invite, go to your Friends tab in the app and tap on the icon in the top right corner of your screen, it's an outline of a person with a plus sign symbol. Find the friend you'd like to add and tap on the circle next to their name to add a check mark. Then tap on the orange box at the bottom of your screen that says "Send invite to 1 friend."
To remove a Friend, go to that Friends page and tap on the "X" to the right of their profile picture. Then just tap on "Remove Friend" to remove them from your friends list.