Guatemala's immigration procedure requires an address to enter the country and sometimes an invitation letter for entrance, so please have in hand your hotel location as well as the invitation letter to attend IWARA. Moreover, Guatemala requires visas for some countries, please check the governmental site: http://www.minex.gob.gt/Visor_Pagina.aspx?PaginaID=21 for a list of citizenship that requires a visa to enter Guatemala.
To get to Antigua Guatemala from Guatemala's international airport "La Aurora'' there are several ways:
1) Shuttle bus or minivan: http://www.transportguatemala.com/book-transportation.php, they have a cost between 15-20 USD but they have limited schedules.
2) Transportation service through the UBER application, where up to 4 people can travel per vehicle. The cost of the trip varies depending on the time the trip is requested, varying between 30-40 USD.
3) Taxi: this is a good option if you come in groups of 3-4 persons, they have a cost of $\sim 35-50$ USD, we strongly recommend to use the company of taxis Amarillo Express: http://www.amarilloexpress.com.
4) Rent a car at the airport between 50-80 USD per day. In order to park in Antigua you need to buy one parking pass every with a cost of around 4 USD per day.
5) We strongly suggest not taking the public transport option, it is really complicated to go to Antigua from the airport and it is not the safest way.
Finally we would like to mention that all places of interest in Antigua are within walking distance.