英语语法问题All the people present at the party were his supporter

定语从句,关系词省略掉了,完整的句子是：
All the people that/who are present at the party were his supporters.
具体如下：
定语从句中关系词省略的6种情形
关系词有时可以省略,但这只限于限制性定语从句中,在非限制性定语从句中,关系词则不可省略.
一、关系代词作宾语时的省略
当关系代词who,whom,which和that在定语从句中用作动词宾语或介词位于句末时的介词宾语时,可以省略.如：
Is there anything (which) you wanted?想要什么东西吗?
Who is the man (that / who / whom) you were talking to?刚才和你讲话的人是谁?
二、关系代词作表语时的省略
当关系代词that在定语从句中用作表语时,可以省略.如：
China is not the country (that) it was.中国已不是过去的中国了.(that作表语)
三、关系代词作宾补时的省略
当关系代词that在定语从句中用作宾语补足时,可以省略.如：
I’m not the madman (that) you thought me.我并不是你所认为的那个疯子.(that作宾语补语)
四、关系副词when的省略
用作时间状语的关系副词when通常不能省略,但有一种特殊情况,即用于day,year,time等少数几个词后时可以省略(也可换成that).如：
That was the year (that) I first went abroad.就是那一年我第一次出国了.
I’ll never forget the day (that) we met.我永远也忘不了我们见面的那一天.
五、关系副词where的省略
用作地点状语的关系副词where通常不能省略,但有一种特殊情况,即用于place,somewhere,anywhere,everywhere,nowhere少数几个词后时可以省略(也可换成that).如：
This is the place (where) they met yesterday.这就是他们昨天碰头的地方.
Have you somewhere (that) I can lie down for an hour?你有没有一个什么地方可以让我躺一个小时?
六、关系副词why的省略
关系副词why通常只用于the reason后引导定语从句,且通常可换成that或for which,均可省略.如：
That’s the reason (why,for which,that) he came.这就是他来的原因.
Give me one reason (why) we should help you.给我举出一个我们应当帮助你的理由

present在这里是形容词，present at the party 做后置定语，这个不是定语从句，只是一个做定语的短语
当然，这个句子也可以改成定语从句
All the people（who were） present at the party were his supporters.

present at the party是后置定语，修饰前面的主语all the people,这是一个简句。如果按你所说的加that或who，就构成一个定语从句。

前面有All后面不能用who，只能用that，that可省

过去分词作定语