#(H_3C)_4C#, has a molecular formula of #C_5H_12#, i.e. it is an isomer of pentane. We could call this #"dimethylpropane"# (am I correct in omitting the numbers, i.e. NOT stating #"2,2-dimethylpropane"#?), but I think it is easier to call this #"neopentane"#. Such so-called trivial names are recognized by most chemists, and most schemes of nomenclature.
The #"neopentyl group"# is #C_5H_11#, e.g. #"neopentyl lithium"# #-=# #Li^(+)""^(-)CH_2C(CH_3)_3#; likewise the #"neosilyl group"# is #(H_3C)_3SiCH_2#, i.e. #C_4SiH_11#..............