MB 9: Never - Because a congregation can not make a voluntary sacrifice except to fill the altar when it is empty, and this is very rare. Therefore we cannot pray Shmoneh Esreh [the Eighteen Blessings, the Silent Prayer] in place of this sacrifice [again, all Amidah prayers are in place of sacrifices --YM], but a single person can pray a voluntary prayer by adding something new, even if he is praying with a congregation.
107:4. (10) One who wants to pray a voluntary prayer must recognize himself to be both zealous and cautious, and be sure of himself that he is capable of concentrating on his prayer from the beginning to the end but if he can not concentrate well this is what is written [Yeshayahu 1:11] "Why do I need all of your sacrifices" - and may we only be capable of concentrating on the three prayers that we are required to say each day.
MB 10: One who wants to pray etc. - meaning even if he is adding something new in his voluntary prayer.