I have found the answer at How do I add contact to a group on Android? to Pentium10
public Uri addToGroup(long personId, long groupId) {
//remove if exists
// this.removeFromGroup(personId, groupId);
ContentValues values = new ContentValues();
values.put(ContactsContract.CommonDataKinds.GroupMembership.RAW_CONTACT_ID,
personId);
values.put(
ContactsContract.CommonDataKinds.GroupMembership.GROUP_ROW_ID,
groupId);
values
.put(
ContactsContract.CommonDataKinds.GroupMembership.MIMETYPE,
ContactsContract.CommonDataKinds.GroupMembership.CONTENT_ITEM_TYPE);
return this.ctx.getContentResolver().insert(
ContactsContract.Data.CONTENT_URI, values);
}