A quick answer

Exactly three groups: yourself, everyone directly above you in the line (your manager, their manager, up to the top) and the administrators of your company. An example: Nora works under Lars, and Lars under Daan. Nora’s actions are then visible to Lars, Daan and the administrators. Her colleagues beside her do not see them, and neither do people in other branches. There are two targeted exceptions: whoever created an action for you can keep following that one action, and the owner of a project sees everything inside that project. See The organization structure and Visibility, security and the archive.

To yourself and to anyone in a layer below yours, in whichever branch. The direct manager of the receiver automatically gets a notification. In a meeting you can additionally assign to all participants. See The organization structure.

It is deactivated, never erased: the name stays in old timelines and reports, but all access stops, including to recurring meetings. See Visibility, security and the archive.

An action is a piece of work that one person ticks off. When you hang actions underneath it, it is a project: the same item, with a list of work below it and a progress bar. The owner of the project sees and manages everything inside it. See Actions and Projects.

Not as the owner; that is deliberate. You ask for postponement on the action itself: write an update, tick Propose new deadline and pick a date with an explanation. The responsible person approves or declines the proposal, and the agreement stays on the action. See Actions.

Finished work stays visible for seven days and then moves to the archive. There everything stays fully preserved, including the timeline; nothing is ever erased. You find it via the Archived status filter on the Actions, Projects and Meetings pages. See Visibility, security and the archive.

Tick Recurring meeting when creating one and choose the rhythm. Every episode then appears by itself, with the same participants, and from the second time onward you see at the start what was discussed the previous time and which actions are still open. See Recurring meetings.

Yes. The pencil next to the title changes the title, the question and the project link, and the participant button adds or removes people, for a series optionally for every next time. See Meetings.

Notifications about your own work come immediately. The daily summary bundles new notifications (each one exactly once) and shows the deadlines that are overdue or due by the end of next week. When there is nothing, no mail is sent. Per person, mail days can also be set. See Overviews and notifications.