The way to insert multiple authors in beamer is not the same as with the usual LaTeX classes. This is a minimal working example of how multiple authors with multiple affiliations can be inserted inside beamer:

% Multiple authors width different affiliations
\documentclass{beamer}

% set a theme
\usetheme{Antibes}

% Title
\title{Multiple author test}

% Authors
\author{author1 \inst{1} \and author2 \inst{1} \and author3 \inst{2}}

% Affiliations
\institute{\inst{1} First institute \and \inst{2} Second
institute}

% Date
\date{\today}

\begin{document}

\maketitle

\end{document}

which produces the following output: