“Most everyone”, meaning “almost everyone”, is a colloquial phrase that became somewhat widespread in spoken American English, but the expression is virtually non-existent in British English and would be considered unnatural by most Britons.
American authors and editors generally advise against using “most everyone” in writing because, strictly speaking, the phrase does not make sense (it should theoretically mean “the majority of everyone”, just like “most people” means “the majority of people”).
In fact, “most” in this context is a contraction of “almost”, just like ’em is a contraction of “them” often used in colloquial speech. However, it is not common to spell the contracted form of “almost” as ’most (with an apostrophe), due to which people often forget that it is colloquial.
To summarize, here are a few examples:
The same advice applies to “most anyone”, “most everybody” and similar phrases containing “most” in place of “almost”: