What were the people complaining of this time?
Rashi: They were complaining that everybody was allowed to enter (the Azarah of) the Ohel Mo'ed, but if someone went a fraction closer than his friends, he was Chayav Misah. 1
Rashbam: They were complaining that, bearing in mind that they were permitted to enter the Mishkan at will, 2 it was difficult to escape the urge to guard it or to assist the Levi'im in their Avodah, which would result in their death.
And what did they mean when they added "ha'Im Tamnu Ligevo'a"?
Rashi: They meant 'Are we Hefker to die'?
Rashi #2 (in Eichah, 3:22): They meant 'Are we finished to die?'