Wedding outfit 😩

Carole
so i am coordinating my step sisters wedding in 2 weeks (i am an assistant manager for weddings as a job).
anyway i am in need of an outfit because i am a guest but i will be running around talking to venders and the caterers. 
what should i wear?! 
i want to wear a dress and possibly heals (ceremony is on the beach) and my boyfriend already has his shirt and black dress pants, i'd also like to kind of match him. 
please post pictures of your dresses and where i can get them. (not a big shopping mall person)
this is his shirt and yes i will iron it at the hotel before