Press the save button to apply edits to your project.
Users should not edit simultaneously
Changes do not automatically appear for all users that have a project open.
When a user saves changes in a project, the bubble in the upper right corner will turn yellow. Everyone with the project open should reload their browser tab to see the updates. Otherwise, the next time you press "save" you are saving an old version of the project that doesn't include the changes your teammate just made.
When multiple people are editing around the same time, it's likely you'll overwrite each other unless everyone is consistently reloading the browser each time someone else makes a save. That's why users should not edit simultaneously.