Q. Can an ovel after shloshim join a group trip that is to travel in Europe to visit and pray at the kvorim of prominent Tzadikim.
A. Shulchan Oruch (Y.D. 380,25) forbids long business trips, as they are very conspicuous and a large group of travelers usually rejoices along the way. (See: Mourning in Halachah 38,6).
Horav Shlomo Miller's Shlit"a opinion is that, if this trip is dedicated for the purpose of visiting and davening by the kvorim of Tzadikim and it is a Torah learning experience too, an ovel is permitted to join.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit"a