Scrummaster or anyone moderating the scrum poker session has to start following web site:
First a session ID must be defined and started. With this ID all members must connect to this session. Then the backlog items can be groomed one after the other. After one item is groomed the scrum master resets all votings and the next voting can be performed. After the session the members could just leave the app or the scrummaster removes all members actively.