Q. Often when I daven in a minyan that says yud gimel midos during tachanun, I am unable to say it together with them, as I am still lagging behind. What is the source that 13 midos need a minyan, and is the fact that there is a minyan present reason enough to say them even if not exactly at the same time.

A. Shulchan Oruch (O.CH. 565, 5) quotes Tshubas Horashbo, that you need a minyan for reciting the 13 midois or “Traits of Compassion” they are a dovor shebekedusho (words of holiness).

Mishna Berurah (ibid. 12) writes that they could be recited without a minyan, if they are articulated as the reading of the Torah and the intention is not for prayer.

Nitey Gavriel (Rosh Hashono 11,10 – quoting Kaf Hachaim and Ben Ish Chai) states that if you begin reciting together with the minyan and they end before, you can still finish your slower recitation.

Horav Shlomo Miller’s Shlit”a opinion is that you can begin saying yud gimel midois as long as some of the congregants are still reciting them.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a