CC Ādi 4.217
āra saba gopī-gaṇa rasopakaraṇa
All the other gopīs help increase the joy of Kṛṣṇa’s pastimes with Rādhārāṇī. The gopīs act as the instruments of Their mutual enjoyment.
It is said that the gopīs are divided into five groups, namely the sakhīs, nitya-sakhīs, prāṇa-sakhīs, priya-sakhīs and parama-preṣṭha-sakhīs. All these fair-complexioned associates of Śrīmatī Rādhārāṇī, the Queen of Vṛndāvana-dhāma, are expert artists in evoking erotic sentiments in Kṛṣṇa. The parama-preṣṭha-sakhīs are eight in number, and in the ecstatic dealings of Kṛṣṇa and Rādhā they side sometimes with Kṛṣṇa and at other times with Rādhārāṇī, just to create a situation in which it appears that they favor one against the other. That makes the exchange of mellows more palatable.