For example, I help run an outdoors and adventure group. Our event types might be "Social", "Hiking", "Mountain Biking","Rock Climbing", "Sailing", etc.
Many members are only interested in Hiking, for example, and hate having to scroll down past numerous other events to find the few they are interested in.

These categories might be related to the keywords used to define a group but it shouldn't be assumed that all keywords correspond to a category.

e.g. for our group we use three keywords "rambling", "walking", "hiking", but the category would probably just be "hiking".

Allow users to select which events they want to see on the events list and allow event hosts/organisers etc to define which categories apply to their event when they are organising events.

You *could* limit each event to 1 category but this might be a limitation for some groups, plus it might be difficult to re-engineer once you limit events to 1 category only. We sometimes do events which offer say mountain biking and hiking, for example, which it would be handy to list under both, but I guess we could always have a "multidisciplinary" event type. We might also want to flag some events as "weekends away".

Allow users to select which event categories they want updates for in emails. Allow users to filter the list of events that appear on the group website when they view it. There should be some indication on the website of the filters in use, with have "edit filter" and "clear filters" buttons as a minimum. Alternatively, you could just have a drop down list which would allow members to select which category of events they wanted to see or leave blank if they want to see everything. E.g. if I selected "Hiking" from the drop down list, it would show only events involving hiking.

You could also add filters for the rsvp status of events i.e. to show events they have rsvped "yes"or "no" to or "haven't RSVPed yet".

The user should be able to filter upcoming and past events on the web site in this way as well as suggestions.

All groups should have a special, default event type of "universal" which allows some events appear on all e-mails etc, e.g. we'd want our members to see things such as club general meetings so would make these "universal" or default and they would appear whatever the user selected (if this annoys users then that's between us and them, not something meetup should dictate to us).