If you have a real life friend who you want to friend on renshuu, ask them for their friend code, a four character sequence of hiragana that you can find from the main menu under Me → Friends.
You only need a friend code if you don’t know how to find their renshuu profile. If you want to friend anyone who publicly appears on the website, like me, just click on their username and press the friend button on their profile page.
Once you have friended someone, you can find their profile again from Me → Friends. From there, you can see their garden if they’ve made it public and any public lists they have made. If you have the leaderboard widget on your dashboard, check the box to display only your friends. They should be there unless they’ve elected not to display their username on the leaderboards.