Make a symlink first, and have tar follow it.
ln -s project-root something-else
tar cjf foo.tar.bz2 something-else/*
/* is necessary to follow the link and select all the contents rather than just tarring the symlink itself. You could alternatively use the
-h option on GNU tar (but this will also follow links inside, which you may not want to do) or the
H option on BSD tar (like on a Mac).
You could even do this from inside project-root:
ln -s . something-else
tar cjf foo.tar.bz2 --exclude=something-else/something-else something-else/*
Unless you explicitly exclude it (as above), the symlink will exist in the tarball too.
Obviously if you are doing this lots of times and don't mind the symlink hanging around it doesn't have to be deleted and recreated each time.