Ordinary modern English uses "whoever", and "whomever" is only used in formal writing, or by people who grew up being taught this was the correct form. If this example of "whomever" is in a book, you may as well throw the book away. Sentences in English are often meaningless without context, and this particular usage is several decades out of date.
A great deal depends on what follows: whoever is nominative - I'll go with whoever invites me; whomever is accusative - I'll go with whomever I first meet. Having said that, I should add that many people, carelessly, or ignorantly, or casually, ignore this distinction, and only use the nominative form.
Whoever is also used generally, in a casual way, often without a verb, to mean any person, just as whatever is used to mean any thing. The WR dictionary includes this use:
informal an unknown or unspecified person: give those to John, or Cathy, or whoever.
Maybe that's what you want, Younghon. As you can see, the choice depends on what is implied in your sentence - what people are calling context.