  We show that two groups of slow two-level atoms in a weak resonant laser
field, are entangled. The considered groups can be separated by a macroscopic
distance, and be parts of a larger atomic ensemble. In a dilute regime, for two
very distant groups of atoms, in a plane wave laser beam, we determine the
maximum attainable entanglement negativity, and a laser intensity below which
they are certainly entangled. They both decrease with increasing distance
between the two groups, but increase with enlarging groups sizes. As a
consequence, for given laser intensity, far separated groups of atoms are
necessarily entangled if they are big enough.
