Fix a natural $\alpha$. Let $n\ge \alpha$ be an integer. Consider the symmetric group $S_{ \alpha+n}$ and its subgroup $S_n$. We consider the group algebra of $S_{ \alpha+n}$ and its subalgebra $\mathbb{ O}[\alpha;n]$ consisting of $S_n$-biinvariant functions, i.e., functions, which are constant on double cosets of $S_{ \alpha+n}$ with respect to $S_n$. We obtain two simple descriptions of $\mathbb{ O}[\alpha;n]$. First, we write explicitly formulas for multiplication in a natural basis (structure constants are Pochhammer symbols). Secondly, we describe this algebra in terms of generators and relations. We also construct an interpolating family of algebras $\mathbb{ O}[\alpha;\nu]$ depending on a complex parameter $\nu$.
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