The welcome message template in Admin > User Interface > Portal Editor is a full template where you can use all templating tags and logic. Here is an example that you can use to show different messages based on usergroup.

<article class="dp-intro-box">
{% if app.user.isMemberOfUsergroup(3) %} Message for usergroup #3
{% elseif app.user.isMemberOfUsergroup(4) %} Message for usergroup #4
{% else %} Message for everyone else {% endif %}
</article>

You can find the usergroup IDs from Admin > CRM > Usergroups, using the small gear icon at the top right of the list.