Answer:
in the picnic activities
Step-by-step explanation:
An adverbial phrase is a group of words that functions as an adverb in a sentence. Many types of word groups can be used as adverbial phrases. Prepositional phrases are often used adverbially, and that's what we have in this sentence.
The prepositional phrase in the picnic activities is an adverbial phrase because it is modifying the verb joined.