Nope, WP multisite will not affect your Google ranking as long as you maintain your previously indexed URL. You already mentioned in the comments that you’ll be using a subfolder installation – that’s great. Though I’d like to add even if you do a subdomain installation (or have a different domain altogether), you can retain your rank via 301 redirects (read this) and informing Google itself of the changes you’ve made via webmaster tools.
Also, since you’re installing multisite I guess you’ll be adding more example.com/sites or even use example.com itself for content – and that’s fine. Just make sure you do not have duplicate content in your multisite, that will get Google to penalize your ranking.