That is very easy since membership to surname projects are open. On your project page, you should have a list of members. You just need to add that person to the list. If you don't yet have a list of members (many don't since they don't have a lot of members), you can just add a section something like this:
=== Members ===
* [[Alexander-7715|Marian Sharp]]
* [[Sharp-10000|Family Member Sharp]]
It will look like:
Members
* Marian Sharp
* Family Member Sharp