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

% set a theme

% 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

% Date




which produces the following output:

Multiple author with different affiliations