check the login items in system preferences>Users & Groups. check skype's own preferences; is it set to open at login?
i'm talking about settings within the skype app. am not familiar with skype for business, but explore preferences, settings, options (possibly with a gear icon), and you'll find it. or look here for more help:
https://support.skype.com/en/faq/FA34552/where-can-i-get-help-with-skype-for-business