A first option is to use comments:
<a href="mailto:#*
*##foreach(${person} in ${people})#*
*#${person.email}#*
*##if($foreach.hasNext)#*
*#,#*
*##end#*
*##end#*
*#?subject=My%20Subject%20Line">Link</a>
Another option is to add a pre-processing that will remove all indenting spaces and carriage returns.
Yet another is to do it as a post-processing.
The way to add this pre- or post- processing is dependent on the context in which you are using Velocity.