Hmmm there are some expressions each of which depends on the context, yes the most common one is اجتمعوا we also have تحالفوا if they were indivisuals we can use احاطوا one cannot refer merely to the fact that they gathered.. we often justify it , why they do so. if there were wicked intentions we use تآمروا or if that person was himslef the wicked one, we use تعانوا also they انضموا for one another to في التصدي if facing. والله اعلم :)

They joined together against me : اجتمعوا ضدي or اجتمعوا معاً ضدي