Saving game: Plants vs Zombies: Garden Warfare II
The game autosaves so you don't have to save. Now I don't know if you're on console or PC but it shouldn't matter. If by this you are referring to an error message then here is a link that could help you.
As for switching users on Xbox, when you are ready to switch accounts press the main Xbox button, then go to the far left to the accounts area. Click the switch accounts button and switch to the requested account. After that PvZ should recognize and switch accounts accordingly.