Different people eat different things and therefore it isnt rude to ask them to bring a dish. Discuss with them what you are intending making and perhaps they can bring something they like from their own tradition and maybe someone will ask them about it.

You never know what you like unless you try things and in this uncertain economic climate it seems wise not to have to cater everything yourself and have it cost you a fortune not to mention all that time you could be doing other things instead of stuck in the kitchen.

Oh yes, I expect everyone to bring a dish at Christmas.  It really irks me if someone thinks that they can just turn up and expect to just sit and eat without lending a hand and expecting everyone else to do the work. And Christmas day is a lot of work for just 1 person.

I would say it really depends on what you're able to afford. If you can afford to feed 20 people without it cutting into your budget or making things difficult for you financially, then do it. If not, they don't be ashamed to ask those coming to contribute. We always brought some kind of covered dish when we went to family's for the holidays. The host supplied the main dishes meat, potatoes, veggie/salad, and dessert. Then everyone else brought whatever they felt like making. It really didn't matter what they brought because we always had about 50 people there and there was always plenty of variety.

Also ask everyone to bring some kind of beverage. If everyone brings a bottle or two of soda or something else to drink, there should be plenty for everyone.

Are the guests your family?  If so then definitely ask everyone to bring a plate, don't forget to specify what is needed.  If the guests are friends and you don't know them all, then ask the ones who are closest to you.  If anybody asks if they can bring a plate, then yes, ask them what they would like to bring and cross it of the list.  Most people realize that 'get-togethers' mean taking a plate and the others figure it out eventually.

Sure ask them to bring a plate I would! You're providing for 20 people and if people had a heart they wouldn't allow you to pay for everything or make everything yourself anyway. Christmas for one family can be expensive enough. Even if you told me not to bring anything I'd still bring over a few kilos of snags and some soft drinks out of respect for our friendship. Family helps family and if they don't then hey it's no loss if they don't turn up =)

Send out every one an invitation and put what you would like them to bring so that you are not ending up with the same foods as it will only get wasted. This is what we do every year and it is so much easier. If you feel a bit rude about it make up some homemade biscuits put in celephane and a card thaking them for coming.