Is this someone who manages a profile that you want to connect your family branch to? Or has someone done some work on one of the profiles that you manage? If it's the first, congratulations on getting connected to the global tree! You can contact the profile manager of any profile by adding a message on their profile page in the box on the right-hand side, or you can send them a private message through a link in the orange box where it says who the profile manager is.
If it's someone who has done some work on one of the profiles you manage, that's fantastic! We love collaboration here at Wikitree. If you'd like to talk to them about it you can see who they are by clicking on the changes tab on your ancestor's profile and see who has been doing the work. From there you can go to their profile page and contact them as above.
Hope this helps,