If $G$ is an affine groupe scheme over some base $S$, you can consider the groupoid $G\rightrightarrows S$. The corresponding prestack $[G\rightrightarrows S]^{pre}$ is (equivalent) to) the prestack of trivial $G$-torsors. The corresponding stack $[G\rightrightarrows S]$ is (equivalent to) the stack of $G$-torsors. So it is right to think of the later as the stackification of the former. You can find details in Laumont and Moret-Bailly's book or in Olsson's book.