Event registration and management like meetup?

Hello all,

I maintain an events calendar for a riding group.

Some of the events are members only, some allow guests, some are formal and pre-paid.

I'm looking for something that would make the calendar more interactive for our members - when I post events people will receive a notice, have the ability to say yes/no/tentative to let us know if they will be there or not.

I think meetup.com is a great example - I'm looking for something like that but on a smaller, single org scale.

At most we may need meetups for different committees (events, safety, etc.) but even that could be maintained on one "meetup".

The payment feature is not required but desired.

Currently the page is done in html/php (with separate apps for calendar, forums, photo albums, etc.) but I'm considering a total overhaul of the site using drupal - the membership piece is an important one.

Thanks in advance,
Regards,
gp