I'm pretty sure it is the first one in my list below - but while I was looking for it I found two more on similar themes: (And do you have any Bruce/Loki recs?)
5 Times Loki Tried To Seduce The Avengers For His Own Ends And One Time He Succeeded Sorta by elarielf Summary: Loki has a new spell! And a great idea! Nothing could possibly go wrong! (Over and over and over again...) http://archiveofourown.org/works/687797
Avengers Undercovers by galaxysoup In which Thor is harassed, the Avengers are easy, and Loki has a deeply unconventional teaching style. http://archiveofourown.org/works/361838
Private Afterparty by lies d Summary: On Steve's birthday, after his friends drink waaay too much to his health, Loki decides to surprise Steve with some shapeshifting shenanigans. http://archiveofourown.org/works/257305
no subject
5 Times Loki Tried To Seduce The Avengers For His Own Ends And One Time He Succeeded Sorta by elarielf
Summary: Loki has a new spell! And a great idea! Nothing could possibly go wrong!
(Over and over and over again...)
http://archiveofourown.org/works/687797
Avengers Undercovers by galaxysoup
In which Thor is harassed, the Avengers are easy, and Loki has a deeply unconventional teaching style.
http://archiveofourown.org/works/361838
Private Afterparty by lies d
Summary: On Steve's birthday, after his friends drink waaay too much to his health, Loki decides to surprise Steve with some shapeshifting shenanigans.
http://archiveofourown.org/works/257305