Grouping nodes in puppet

There's two ways:

1) Multiple node names in one line:

node 'puppet-client1', 'puppet-client2' {
    include testmodule
}

2) Nodes can be defined with regexes, e.g.:

node /^puppet-client[0-9]+/{
  include testmodule
}

but that depends on a suitable host naming scheme.

You can see the official documentation for node definitions.