占领 'to occupy (militarily)' +
者 'one who…'. The -
者 suffix turns a verb into 'the one doing it' — see
学者 (xuézhě, 'scholar / one who studies'),
记者 (jìzhě, 'reporter'),
译者 (yìzhě, 'translator').
占领者 is almost always military or political; for 'occupant of a building', use
居住者 (jūzhùzhě) or
住户 (zhùhù).