首先,这是个 the more.the more. 的句式（还有一个the more 没写出来）

the more people they will benifit 意思是：他们将使更多的人受益

benifit：使受益
不懂再问全句是the more museums open their doors to the public for free，the more people they will benifit 。句子中的they可以省吗不可以首先benifit：使受益，是使役动词，去掉they 的话应改为：the more people will benifit from them(them指museums)其次，they指代的是 museums ，they 是主语，they使people受益