\documentclass[a4paper]{memoir}
\usepackage{xparse,amsmath}
\ExplSyntaxOn
\clist_new:N \l_my_clist
\NewDocumentCommand\tsub{ m }{
\sb{
\clist_clear:N \l_my_clist
\clist_set:Nn \l_my_clist {#1}
\clist_pop:NN \l_my_clist \l_tmp
\textup{\l_tmp}
\ifnum\clist_count:N \l_my_clist>0
,
\clist_use:Nn \l_my_clist{,}
\fi
}
}
\ExplSyntaxOff
\begin{document}
\[
X\tsub{max} \qquad X\tsub{max,d,t,t}
\]
\end{document}