Ahhh... here we go again... but i'll give my suggestions based on you as a canon user.
On the basis of max 2 camera bodies...
1st Camera (1.6x):
Zooms: Tamron 17-50 f/2.8, EF-S 17-55 f/2.8 IS USM, EF-S 17-85 f4-5.6 IS USM or similar.
2nd Camera (1.6x):
Primes: EF 50 mm f/1.4 USM, EF 85 mm f/1.8 USM or similar primes. I find 85 mm too tight but some like it.
Zooms: EF 70-200 f/2.8L IS USM or equivalent, best lens for do it all telephoto.
1st Camera (FF):
Zooms: EF 28-70 f/2.8L USM or equivalent, if you like wide can try EF 16-35 f/2.8L USM.
Primes: EF 28 f/1.8 USM, EF 24 f/1.4L USM, EF 35 f/1.4L USM or equivalent wide angle prime, the capture-it-all-in-a-shot camera. Primes due to no distortion at such wide angles.
2nd Camera (FF):
Primes: EF 85 mm f/1.8 USM, EF 85 mm f1.2L USM (Best price as well!) or similar such as 100 mm, 135 mm (too tight in HDB for my taste).
Zooms: 70-200 f/2.8.... easy choice or similar if no budget.
My setup preferred and current setup is 28 mm f/1.8 on 5D2 (Body 1) and 85 mm f1.2 on 5D1 (Body 2).
best lens is the one you got in your hand right now + flash...
sir, toilet is on the left walk till the end and you'll see the sign... ATM is at the basement nearby the escalator... Foodcourt is at 4th floor you can go up by the lift is faster... ;p by the way sir, are you in the same shopping mall as i am?? ;p
If you are sticking to your current 500D + 18-55 config, consider borrowing/ renting a flash to get at least a decent number of sharp photos especially indoors or during the banquet.
Whether you should invest in additional lenses be they primes or zooms should be dependent on whether you are a casual or paid photographer on that day. Of course if you are a not paid but very enthusiastic to take the opportunity to learn how to use new lenses in a real situation, good on you. There are many helpful suggestions given above and in other threads, find the lens that rocks your mount.
For a start your 18-55 is good enough for table/group/gown shots, plus borrow or rent a 50 f/1.8 for portraits and a mid tele prime/zoom like the 85 f/1.8 or 70-200 f/2.8 for catching moments from afar or in church/ROM situations.