Given a countable group $X$ we study the algebraic structure of its superextension $lambda(X)$. This is a right-topological semigroup consisting of all maximal linked systems on $X$ endowed with the operation $$mathcal Acircmathcal B={Csubset X:{xin X:x^{-1}Cinmathcal B}inmathcal A}$$ that extends the group operation of $X$. We show that the subsemigroup $lambda^circ(X)$ of free maximal linked systems contains an open dense subset of right cancelable elements. Also we prove that the topological center of $lambda(X)$ coincides with the subsemigroup $lambda^bullet(X)$ of all maximal linked systems with finite support. This result is applied to show that the algebraic center of $lambda(X)$ coincides with the algebraic center of $X$ provided $X$ is countably infinite. On the other hand, for finite groups $X$ of order $3le|X|le5$ the algebraic center of $lambda(X)$ is strictly larger than the algebraic center of $X$.
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