  We synthesize homogeneous compositions of NaLu$_x$Yb$_{1-x}$Se$_2$,
connecting non-magnetic NaLuSe$_2$ to the triangular lattice spin liquid
candidate NaYbSe$_2$. Thermal and magnetic properties are studied as the system
evolves from one with dilute magnetic defects to one of a dense magnetic
lattice. The field and temperature dependent heat capacity show the carriers of
entropy crossover from isolated magnetic ions to a correlated lattice borne
from spin dimers. For the dilute system we estimate the single ion anisotropy
$(g_\perp/g_\parallel =3.13)$ and also the dimer exchange couplings
$J_\parallel(=5.4$~K) and $J_\perp(=9.6$~K), in order to draw comparison to the
half-doped and full magnetic compounds.
