Hello
You may change the default group on My Account -> Privacy Settings -> Default Privacy Group block. But this change with be done for current member only.
If you want that Friends was a privacy setting selected by default for all newly joined members then you need to do the following. I will give an example for "Who can view my profile page".
1. You need to go to Dolphin database using phpMyAdmin.
2. Open structure of Profiles table and edit "allow_view_to" field.
3. Change the default value 3 to 5 and save it.
So, all newly goined members will have Friends in "Who can view my profile page". You may check it on Profile Edit page,
If you need you may do the same for the other content like Albums (sys_albums table -> AllowAlbumView setting) and so on.