When you are having a party of any size, type, or scale, issuing the invitations can be a vital decision for its success. Invitations that you send to friends and family can be done by phone, mail, email, or Facebook. Making the invitation by phone makes it really personal, and also gives you instant feedback on whether they are interested in attending or not.
As a guest, when it happens that you receive a written invitation to an even, you should reply at once. What you do not want to do is simply not respond, but still show up to the event. This sort of shock will be an embarrassment to your host, as they may not have a seat for you, or enough food to go around. Keep in mind that when you get an invitation you should decline the offer or accept it with promptness, so that the person that invited you can adjust their plans accordingly.
It is smart thinking to invite your guests at least two weeks ahead of time. If you are planning a large event, it is appropriate to start calling a month prior. If you have tried sending out invitations first and then waiting for a reply you will be able attest that the waiting time can be far to long for a reply. To get around this, try calling first to get an immediate yes or no, and then follow up with a postcard with the date and time and location of the event as a quick and handy reminder for your guests.
You may run into the situation that when you invite a couple, only half of the couple can attend. This can occur when one person is on a business trip and the other is at home, or if one as a prior engagement that the other is not obligated to. At this point you obviously must invite the individual that can make it, even if it will throw your seating numbers off.
When you are extending the invitation by phone you must give the person the date and the time as a minimum of information. If you are inviting someone that is a new acquaintance of yours you might want to mention some other guests who are mutual friends that will be attending so they can make a better decision as to whether they can attend or not.
If you are being asked to a party, it is rude to inquire as to who else is coming. The host may offer this information to you, but you should not solicit it from them. You should not base your decision of whether or not you will be attending on who will be there and who will not. If you want to go you should give a firm yes, and if you do not want to go you should politely decline, and stick to your decision.