robots.txt

You can use the internal Elgg feature on this page: /admin/configure_utilities/robots

Or just create robots.txt and upload it to your Elgg root installed directory.

You can use my snippet for this.

sitemap.xml

There're some Elgg plugins but I'm not sure if they work. Use them at your own risk.

We've Sitemap plugin on our marketplace also.

We and our customers use it on all sites.