I think they both work very well and sound natural, but they have different meanings -- 'they' are going to 2 different places! In the first option, you say they are coming (to wherever you are) and in the second option you say they will be there at the restaurant) soon. Perhaps the second should say "they'll soon be HERE".

The second one for sure!

I would say 2, but the word order is unusual. It's correct, but old fashioned. "I've invited my friends to the restaurant, and they'll be here/there soon."