This is the 308th most frequent Slovenian word.
skupini
"Skupini" is the dative or locative singular form, or nominative dual form of the Slovenian word "skupina," meaning "group."
Here, "skupini" is in the dative case because it indicates the indirect object of the verb "pridružiti se" (to join).
Pridružila se je plesni skupini.
She joined the dance group.
In this sentence, "skupini" is in the dative case, as it shows the entity receiving the action, related to which the microphone is connected.
Priključila je mikrofon eni od dveh skupini.
She connected the microphone to one of the two groups.
Here, "skupini" is in the locative case following "v" (in), indicating the group in which the action of participation occurs.