Permissions are broken up by Group and Person, meaning one person in a particular group can have totally different access levels to another person in the same group.
Typically permissions will be the same for someone for any type of event - eg. they will be able to 'modify' any event information for a group, although they can also be customised by event-type. This means someone who might be able to modify event details for 'Sporting' (event-type) events, can only 'view' (or have no access at all) to 'Community' events.
By default, new users only get 'View' access to events within their own groups.
People need to have their account enabled first before they can login - until then, they won't be able to access the system.