can I use skype for business (office 365) for helpdesk? So a user can start a chat with helpdesk and one member of the helpdesk group answers (ACD / response group functionality).
Not directly, a call queue in Office 365 is for audio only. You can't use either a distribution group or Office 365 group to distribute the chats as all the chats will go to all the agents.
You could have a look at some of the webbased chat services withfederation to Skype for Business, or consider deploying an on-prem service to handle the distribution of chats.