First thing you must do is verify they have registered on SoliKick - You will also need to know what their name is on their profile in order to add them.
There are a couple of ways to do this - the most comprehensive way is to go to the top menu bar and select "Members":
From there, you can use the search fields on the left to find your friend:
On the bottom right, click the 3 lines "More" - From there select "People" - You will need to type in their name as it appears on their profile.
Try searching by your friend's last name