If I am writing an event invite for a [Company Name] Christmas Beach Party, should the Beach Party be capitalized?
As the title of an event there is certainly scope for using title case! i.e. capitalising the first letter of each word, since you're free from articles, prepositions and conjunctions there isn't too much grey area to draw in confusion.
Where there are conflicting or vague rules regarding such issues, my general approach is to go with how the end result looks and feels Personally I would lean towards capitalising everything simply because it looks more pleasing. (and there is certainly leeway in the rules to allow for that)
You'd capitalize "beach party" if the entire phrase is a proper name, because it's an official event:
You are invited to the WidgetCo Christmas Beach Party! This is being held across the street from the restaurant where we had last year's WidgetCo Christmas Karaoke Night and down the block from the inaugural WidgetCo Christmas Weenie Roast.
If you're just talking about a general event which doesn't have a formal name, keep it lowercase:
You are invited to the WidgetCo Christmas beach party! This is being held at the same place as last year's WidgetCo Halloween beach party, two sandbars over from the inaugural WidgetCo Arbor Day beach party.