Once you have 10 friends showing on the ingame friend leaderboard for a table, your achievement should unlock. However, I couldn't get the achievement to pop until I tried the following.
If you already have 10 or more friends or if you need more, you can simply go to the Pinball Wizard leaderboard on the far right of the first ingame menu, keep scrolling right and you'll eventually see it. Click on any player on there to open their Xbox profile and click add as friend and it should say they have been added as a friend. Keep doing this until the achievement pops.
In the event the leaderboard isn't updating with the newly friends, simply completely close Pinball FX 2 with the X icon in the top right corner and open Pinball FX 2 again and it should update the leaderboard with your new friends.
You can also update the leaderboard by playing any table and losing your balls and it will tally up friends scores.