On the Privacy tab of any profile, if you scroll to the bottom of the profile, it will have a link to 'click her to request to join the Trusted List'. When you do that, just mention that it is your relative or how you are related and that you would like to be added to the Trusted List as a co-Manager. The person may then do that and you would get an email if you were added to the Trusted List.
If the profile is open, you can add sources and information to the profile, which is part of our 'collaboration' on wikitree of our 'shared one tree'.