Q. Is there a problem for a Beis Din in a time of need to take a group of gerim and do the tevila and kabalath mitzvoth together?

A. See question above where Poskim maintain that there is at least a mitzva “kiyumis” in converting a suitable proselyte.
Although our forefathers underwent a process of gerus at Sinai as Rambam explains, and it was done in mass, traditionally it is carried out with particular and personal care, on an individual basis.
Devar Moshe (Yebamos p. 406) debates whether gerus is a ma’ase Beis Din similar to retziah and two individuals cannot be processed together due to the principle of “Ein oisin mitzvos chavilos”, or denigrating mitzvos by bundling many together. (Sota 8a, Mishne Lemelech H. Avadim 3: 9, Avnei Mishpot p. 42 n. 9))
Horav Shlomo Miller’s Shlit’a opinion is that in some cases, a family with several small children, may do the tevila with one of the parents and there is no issue of “Ein oisin mitzvos chavilos.” Otherwise, we should abstain from a group gerus, as the connotation today is very negative.

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