I don't know much at all about UUPA, but I am assistant organizer with my local poly meetup.
It's a big group in a growing metro, so we try to fill the calendar with a hodgepodge of stuff and cover the whole region evenly.
Mostly social meetups---that brings out the most interest by far. Some "regular" gatherings at coffee shops & bars, and then some big special things (like a holiday party, etc).
But we do scatter some focused poly discussions & forums in, and if you spread them out over time and announce in advance, they will then have a good turn out as well.
Variety is key as well as keeping things fun. That helps.
We do it thru meetup--used to be a email list but that wasn't very functional for organizing, setting up locations, RSVPs & calendars, etc. Meetup makes that easier. (Also it's a go-to site for people when they move to a new city---they always look for meetups first.) Tends to help attract more young adults, too, especially if you duplicate some of the event info on facebook.
One person mentioned overlap with kink/bdsm meets. (I know groups in neighboring regions have done that.) So far, while I know folks into that scene are in our group, they haven't yet asked for any meetups oriented to that---they use fetlife mostly instead. Ours so far has been just-poly oriented, and that seems to get the widest interest, and definitely lots of diversity in types of poly styles (open poly to polyfidelity, vanilla to kinky, and all in between).