I have version 2.1.7 right now. Is it true that if I upgrade to the latest version I will lose all posts and people?

No, you won't. Just remember to backup your database before upgrading!

Absolutely not. o.o

Every version of IPB comes with an upgrade utility. You'd simply upload the new files over the old ones and run the upgrade script, all forums/categories, member accounts, posts and topics, etc, all of it would be fine.

Thanks