We have been to quite a few adult fancy dress parties hosted by some friends of ours . I recommend that you find out from your hosts as to what they will be expecting as there are adult parties and adult parties were sex could happen .
Ours have been mainly the former whereby there will be no kids present and the costumes could be a little more risque ie female leg/thighs on show or naked male torsos for example there have been one or two wantobe tarzans, St Trinian style girls and sometimes a topless male butler serving drinks. I just hope they havn't got me lined up to perform the latter as they have ben quite complementry of late since I started working out!.Anyway my Mrs has worn things like some of the LH costumes like the police woman dress which is just about decent enoigh to be worn at a party in our opinion although she did team it up with fishnet stockings and wore modesty shorts underneath.We have been to a halloween fancy dress party dressed as Mr and Mrs Beettlejuice . Again my wifes costume was based around a short skirt with tights but this time white . Activities tend to be things like stripping games ( only down to underwear ) or a twist of the pin the tail to the donkey game which was a guy pin up and it wasn't a tail to pin !
They have also hosted a more kinkier type part evening for charity raising . Althoughh around dinner everyone would have been more formly dressed and then followed a quiz. The losing couple would then throw a dice to seal their fate. If the dice gods wern't on your side the losing couple would be required to be naked below the waist and bent over a stool to recieve 6 strokes of the cane and administered by the hosts wearing their dom and dominatrix type costumes .All good humoured I hasten to add. I took advice from here and decisded not to go as it could have mortified my partner should we have come last at the quiz.
If you want my view on the costume to wear , then perhaps something a little conservative at first as you could always dress down later when you have tested the water. What you don't want to be IMO for a first timer is to be the centre of attention unless you are really comfortable with it. Only you can decide on the latter .
Whatever you decide I am sure that you will enjoy yourself .