So Bhaktivinoda Ṭhākura is an ideal guru. He was not a sannyāsī; he was gṛhastha, householder, living with family, wife, children. Still, he was guru. So anyone can become guru. Not that a sannyāsī can become guru. A householder also can become guru, provided he knows the science. Caitanya Mahāprabhu, when He was talking with Rāmānanda Rāya . . .
Caitanya Mahāprabhu was a sannyāsī, very highly born in brahmin family, very learned scholar. So He was talking with Rāmānanda Rāya, a gṛhastha, governor of Madras. And He was questioning, and Rāmānanda Rāya was answering. That means he was taking the part of guru, and Caitanya Mahāprabhu was taking the part of a disciple.