I've hosted and attended a number of beer tastings amongst my group of friends. We're all big beer nuts, and our tastings are generally focused on a particular style or genre - belgians, pale ales, imperial stouts, bocks, etc. You could also do tasting based on country or region - belgium, germany, england, etc.
We ask for everyone attending to bring at least 2 quality beers (no BMC), with enough to fill a sample glass for every attendee (3-4oz). Attendees are asked to email the host their selections prior to the event, in order to prevent duplicates.
Here's the end result of our Belgian tasting
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