can you be a little more specific.
Users here in the community?
Users who submitted info into FamilySearch Family Tree?
what user are you trying to contact? and based on what?
You can only contact those you make entries into the tree/profile of an ancestor. Open the ancestors profile and you can either click on any entry/fact to see the "author" in blue, click on their name and a profile will come up with their name, possible entry of an email address and the prompt to send a message. At this point you can either privately email them directly or send a message via FamilySearch. The other option is on the right side of the profile is the "Change Log" This is a comprehensive list and when you click on "See more" you get a large group/list of submitters to then click on their names and do the above contact options. Please remember if sending a private email vs. FS message to include identifier info about the ancestor, ID # etc.