You can use the Awesome library with a lot of icons. This an example how o it has worked for me:
<!-- Delete Button -->
<p:commandLink style="padding: .3em 1em"
                      styleClass="fa fa-trash"      
                      process=":formDataTable:customers"                                       
                      update=":formDataTable:customers">
    <p:confirm header="Delete Confirmation" message="Are you sure?" icon="ui-icon-alert" />
    <p:collector value="#{customer}" removeFrom="#{customersController.customers}" unique="true"/>
</p:commandLink>
In your case it would be like this:
<p:commandLink 
     styleClass="fa fa-user-plus"
     action="#{bean.doSomething}">
     <h:outputText value="Add" />   
</p:commandLink>
Just be sure to the the web.xml with primefaces.FONT_AWESOME to true like this:
<!-- web.xml -->
 <context-param>
    <param-name>primefaces.FONT_AWESOME</param-name>
    <param-value>true</param-value>
 </context-param>
I hope it helps somebody else!