\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand{\definearray}{ m m }
{
\clist_new:c { g_vizipok_#1_clist }
\clist_gset:cn { g_vizipok_#1_clist } { #2 }
\cs_new:cpn { #1 } ##1
{
\tl_if_blank:nTF {##1}
{ \int_eval:n { \clist_length:c { g_vizipok_#1_clist } } }
{ \clist_item:cn { g_vizipok_#1_clist } { ##1 } }
}
}
\ExplSyntaxOff
\definearray{names}{Katie,Frank,Laura,Joe}
\begin{document}
\names{}\par\names{2}
\end{document}