I would choose Bens structure for the guest info:

struct GuestList{
int GuestID; // The guest's ID number
char NameFamily; / Surname
char NameFirst; // Firstname
char AddrNumber; // Char, so house names can be used
char AddrRoad; // First address line (ie, road)
char AddrTown; // 2nd address line (ie, town)
char AddrPostCode; // Postalcode
int Veggie; // 1=Veggie, 0=Not
int RSVPYesNo; // 1=Coming, 0=Not
int RSVPStatus; // 1=RSVPed, 0=Awaiting Reply
}

Kapriels food choice function, as it gets to the point:

Void FoodChoice(Gname/ID)
/* Input Veggie/non-vegie*/
/*output ListV/ListNV*/
//Inputs if guest is veggie or not; Outputs both Veggie Guest List and Non-veggie Guestlist

Lloyds RSVP function:

void Process RSVP (guest)
/*input: Guest, invite ID & reply*/
/*output: void */
/*Comment: Function will take the guest id and compare this to the reply and if true will print to guest list*/
Int Food(guest)

I quite liked Mohds suggestion of a structure to state what type fo music people wanted:

//Structure genre of music played--------------------------------------
typedef struct {
char jz; //for jazz
char alt; //alternative
char rnb // for the R&B music
char evrgreen //for old-skool song
}genre_music;//---------------------------------------------------------

Another of Mohds:

/*--------------------------------------------------------------------*/
funtion 3 (Comments status)
/*inputs: import the data from the data array*/
/*outputs: received the data and produce the sentences*/
/*comments: will know special request from guest, either foods,music,etc*/
/*----------------------------------------------------------------------*/

Don't know why I didn't mention this one of Lloyds creation earlier:

Void Read/Send( Guest);
/*input: Guest list */
/*output : Void*/
/*comments: Function will take the guest details and print invites containing their details as well as a generic set of information */

Kapriel had a good idea here - finding out the cost (to the host) for each person:

Void Price(Gname/ID)
/* Input: AvPriceVeg, AvPriceNveg*/
/* Output: PriceListVeg, PriceListNveg, PriceListGen*/
//Inputs the average price per person for both meal and entertainment expenses; Outputs Price lists for Veggies, Nonvegies and for Everyone all together//

And finally, a useful guest list courtesy of Ben:

void PrintGuestList();
/*Input: From the Guest list */
/*Output: Void */
/*Comments: Prints out a list of the details for the guests in the party-owner format (eg, guestlist on door) */