$group=Db::name('auth_group')->select();
foreach ($group as $v){
    $v['auth_group_access']=Db::name('auth_group_access')->where('group_id',$v['id'])->select();
    foreach($v['auth_group_access'] as $k1=>$v1){
        $v1['user']=Db::name('user')->field('truename')->where('uid',$v1['uid'])->find();

        $v['auth_group_access'][$k1]=$v1;
    }

    $data['group'][]=$v;
}

结果形式：

 "group" => array:11 [▼
    0 => array:5 [▼
      "id" => 1
      "title" => "aaa"    
      "auth_group_access" => think\Collection {#78 ▼
          0 => array:4 [▼
            "id" => 1
            "group_id" => 1
            "uid" => 1
            "user" => array:1 [▼
              　　 "truename" => "aaa"
            ]
          ]
　　　　　]
      }

$group=Db::name('auth_group')->select();
foreach ($group as $v){
            $auth_group_access=Db::name('auth_group_access')->where('group_id',$v['id'])->select();
            $a='';
            $b='';
            foreach($auth_group_access as $k1=>$v1){
                $user=Db::name('user')->field('truename')->where('uid',$v1['uid'])->find();
                $a=$a.$v1['uid'].',';
                $b=$b.$user['truename'].',';
            }
            $v['auth_group_access']['uid']=$a;
            $v['auth_group_access']['truename']=$b;
            $data['group'][]=$v;
        }

结果：

"group" => array:11 [▼
    0 => array:5 [▶]
    1 => array:5 [▼
      "id" => 4
      "title" => "ttt"     
      "auth_group_access" => array:2 [▼
        "uid" => "73,101"
       "truename" => "aaa,bbb"
      ]
    ]