All the members in a particular homologous series have the same. . . . . . 

[Step 1]:
Homologous series may be defined as “a series of similarly constituted compounds  in which the members possess the same functional group and have similar chemical  characteristics”.
[Step 2]:
Therefore the members in homologous series have the same general formula.
For example CH4 and C2H6 are same family. But they have different molecular  formula, molecular weight and density.
Both have same general formula ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+2}$.