If it's a profile someone else manages, and it's an 'open' profile, you can add information. You can check the differences in 'privacy' levels here: http://www.wikitree.com/wiki/Privacy which describes who can edit data on less public profiles as well.
You can also contact the profile manager and ask to be added to the Trusted List, (include your email address in the correspondance) or just share your information.
On any profile page, on the left side under the persons details, you will see a line like this:
Profile manager: ________ 
[send private message], click the
[send private message] link to email the manager directly. You might find a long lost cousin in the process!
More about the Trusted List:
http://www.wikitree.com/wiki/Trusted_List
