"Can you join Duncan and me in the field?" is correct.
Use "I" when it is the subject of a verb and use "me" when it is the object of a verb. In the above sentence, "Duncan and me" is the object of the verb "join."
One way to check if you have the correct pronoun is to take out the other noun:
"Can you join I in the field?" is not correct.
"Can you join me in the field?" is correct.
I hope this helps.