Many people who have been to the park _____（返回）with their friends.

return
因为前面用了现在完成时have been,所以return用一般现在时.have been 后面不是加doing或done的么、课本里常用的一个表达方式，Have you ever been to......你有没有去过什么地方。说去过哪里时用have been to就行，不用额外加动词。可是return是动词、Many people who have been to the park _____（返回）with their friends.很多人去过这个公园的都是和他们的朋友们一起回来的 who have been to the park 就从句。主干是Many people return with their friends.所以return和who have been to the park是相互独立的。