Go to their profile page (click on their name anywhere you see it like in daily report, a forum post, etc...).
To the right of their name near the top of the page you will see a link "support".
Click it and voila!
I found people in forums on the site, for instance there was one for people who eat around 1800 calories, I basically support everyone from in there, also there was one for people from Manchester, UK (my home town) I support them too. To actually support them you just need to click their name and then their profile will come up, there you just need to click support at the top of the page as explained in the above post. I have started supporting you, hope you dont mind :)
Go to your profile - at the end of the web address, delete your username and type in your friend's.
On their profile, click Support.