There definitely is a generational gap... a definite 10 year grouping, but probably even a bit more of a breakdown into 5 year groups or so-- those who were active at the same time tend to want to hang with their own.
With the exception of formal rituals and such (with privacy concerns and uninitiated respect), it's probably easier to bring alumni in if their families can also be included. If alumni feel like hanging with the actives is like spending time with their kids-- perhaps it's an event to celebrate that. Bring the kids, introduce them to positive aspects of greek life, and bring the brother (or sister) - hood into a larger, stronger extended family. Our lives as brothers and sisters are not limited by the Greek community-- our families are also influenced by them through us.
Things like BBQ's, sports events, house cleanings/updates (do some work, and then have a potluck, etc) and service/philanthropy events are often ones that can be "the more the merrier" events. If families are welcome, then it's easier to get alumni to come-- especially if they have kids that they'd have to arrange for.
When I was an active, we had our annual end-of-year picnic, which was originally set up as a way for alumni to catch up with the actives at least once per year. It was casual, a complete potluck, usually including a softball game and just relaxing and chatting. One year, kids started coming with the alumnae moms. And then the next year, husbands... and from there, it just became a good time to see the alumnae *beyond* the names and stories we'd heard about their active years. We saw the people who now were important in their lives and got to celebrate those joys with them.
~ Mel.