Q. If nine are davening and one who already davened will daven again as Nedava (voluntary praying) is that called Asara Mispallelim (a full Minyan)?

A. Mibeis Yisroel (50 p. 65) rules that it would count as a minyan.
Shulchan Aruch (O.H. 107; 1- 4) details the many complexities and difficulties that one must comply with in order to daven a Tefilas Nedava.

Biur Halacha (ibid) quotes Chaye Adam’s ruling that in our days one should abstain from davening such Tefila since it is unlikely that one may fulfill all those requirements. He mentions that maybe one in a thousand, may comply with them, and that was in his days.

Horav Yitzchok Zilberstein Shlit’a addressees the shaila and quotes the Chaye Adam and agrees with him (Vavei Haamudim p.56). He also quotes a similar opinion in Ketzos Hashulchan He maintains that one cannot form a minyan by using a voluntary prayer
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Rabbi A. Bartfeld as revised by Horav Shlomo Miller, Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu and Horav Kalman Ochs Shlit’a