How can I contact someone who’s posted information about my ancestors
The username of the person who entered the data will likely appear in blue. Click on that, and you will have a chance to send them a message.
If the person does not reply, and you know the researcher's name, google their name and the ancestors surname, and you may find a place online where they posted a querry, and you can reply to his or her querry.
Welcome to the "Community.FamilySearch" Forum.
I am juts another 'lowly' User/Patron ...
Further to what 'Tom' has already proffered ...
Here is a "Knowledge Article" in 'FamilySearch':
And, here is another ...
I hope these may also help.
All of these comments are good. To slightly correct the comment by Tom Robertshaw...The name that you see next to information in the Family Tree that indicates the person who made and addition or change is a "contact" name. This is different than the username that you use when you sign in to your account.
You can change both your username and your contact name by signing in to your FamilySearch account, click your Display name in the top right corner, click Settings. On the Account page you can edit the Username or the Contact name. Because other people will see the contact name you can make it incognito if you want to.
Thanks, Tom, and others for your good answers.