Please help with menu suggestions for a stand up dinner party for forty to fifty people.

Little shots of cold soup? Cauliflower, vichysoisse, or maybe an avocado gazpacho?

By stand up dinner party assuming you mean buffet. Best article ever. http://www.bonappetit.com/entertaining-style/parties/article/buffet-menu-ideas

Whatever items you end up picking, make them "one-bite" pieces. Your guests will appreciate not having half of something squirt down the front of the good clothes as they try to bite into a big canapé !

Since it Summer why not do different food on skewers? They are easy passed appetizers and a yakitori party would be great with drinks! Here is my recipe for pork bulgogi that would be super portable for a stand up dinner and they are affordable and easy! https://food52.com/recipes/36459-pork-bulgogi-yakitori

Individual hand pies/tarlettes, savory choux, filled gougere, frico cups, small tartines, savory cheesecakes, gazpacho/soups in little shot glasses, roasted shrimp with dipping sauces, savory puff pastry pockets.

I'm not sure how much you need done in advance and if you can do anything last minute, but those are my suggestions. If I have any more, I'll let you know :)