I'd like to let different users (call them jack & jane) launch mongod using the same dbpath (call it /datadir). Here's what I try:
1) chown all the files in /datadir to jack:mongogroup
2) chmod -R g+rw /datadir
3) useradd jack and jane to mongogroup.

When I launch mongod --dbpath=/datadir as jack, things are fine. But when I try to relaunch mongod as jane, I get errors like this:
Wed Nov 7 00:30:43 [initandlisten] warning couldn't write to / rename file /datadir/journal/prealloc.0: couldn't open file /datadir/journal/prealloc.0 for writing errno:1 Operation not permitted
Wed Nov 7 00:30:43 [initandlisten] couldn't open /datadir/sample.ns errno:1 Operation not permitted

This doesn't seem to be (simply) a permissions issue. If I make a mistake on permissions, I get "errno:13 Permission denied", instead of "errno:1 Operation not permitted". Also, jane can read/write test files in that directory as expected (aside from launching mongod).