Database structure for users, events and event lists
Based on your comment above, it sounds like when you said everything was stored in separate "databases", you actually meant to say "tables". I'm going to work with that assumption...

I'd suggest a simple table that stores who's been invited to which event, and what their response was.


invitations
-----------
  id (primary key for the table)
  user_ID (foreign key to users.ID)
  event_ID (foreign key to events.ID)
  response (could be a simple string, or an ID to a table of possible responses, such as "attending", "not attending", "unsure"...)
  date_sent (a date showing when the invitation was sent)
  respond_by_date (date that a user must response by)
  mode_sent (was the invitation sent by email, text message, other electronic message, regular mail, semaphore flags, etc...)


The above structure will keep track of which user was invited to which event. It will also keep track of the user's response. Other fields such as date_sent aren't necessary, but could be useful for other purposes. Data for this might look like:


invitations
-----------
id   |  user_id  | event_id   | response  |  date_sent  | respond_by_date  | mode_sent
--------------------------------------------------------------------------------------
1    | 123       | 5          | attending |  2014-12-01 | 2014-12-05       | email
2    | 456       | 5          | not attnd |  2014-12-01 | 2014-12-05       | email
3    | 456       | 7          | attend+1  |  2015-01-01 | 2014-12-02       | semaphore flags



(This assumes that the user and event tables already exist)

Would this work for your purposes?