1. In "Contacts", click one friend and get into his/her profile
2. Click "Set Remark and Tag"
3. At "Tags", type a tag for him/her. Like "Family", or "Classmates" etc.
For example, set "friend_a" "friend_b" ... with same tag "Classmates".
Set "friend_d", "frend_e" ... with tag "KidsPlayDate".
4. Then you have several groups for your friends, group for "Classmates", group for "KidsPlayDate" etc.