Invitation System - Design Help for Newbie

Hello there,

I just started a project using RoR. Was a C# / .NET programmer before
jumping into this project.

Here's my project's details:

Architect a website that has users, projects, events, and groups. The
site has a simple invitation system. Any user can be part of a
project, an event, or a group. Users, Projects, events, and groups
don't need to contain any information and could just be a primary key
(keep things very basic). Come up with a way to model this in the
database and in your model.

Add the ability to send a note along with an invitation (the note is
attached to the invitation). Set up some test data for users,
projects, events, and groups then create control logic for a user to
invite another user to projects, events, and groups. The output of
the invite could be as simple as 'user 5 has invited user 1 to project
3 – “this is my test note”' or 'user 2 has invited user 3 to event 1
– “hey join my project”'.

Question(s):

(1) How would the database look if there was a way for users to accept
an invite to a group, event, project?

(2) Is the ability to add notes (text) with an invitation easy using
ActionMail?

(3) What would be the best way to go about this?

Any help, tips and suggestions are greatly appreciated!

-Mike