There are two options for synchronization, they can be toggled in the Share tab, View section.
Work Alone
Work alone (current slide has orange border; if you switch to another slide it is only done locally - please use this mode when adding content and switch back to follow presentation when done)
Follow Presentation
Follow presentation (current slide has green border; changes the slide globally - the host uses this mode to guide the presentation and all other participants use it to follow the presentation).