You start collaboration WITH someone that you want to work with. From their profile you will see, in to the right of their name a link that says "Request To Collaborate". You'll have the option to explain WHY you're requesting this. It is HIGHLY recommended that you do so, as people are more likely to agree, if you do.
As to locked profiles, you'll see a line that says "Curated By" with the name of the relevant Curator. You can send them a message directly, or start a public discussion about the profile.
James, I curate some locked profiles and am happy to take requests to add data or do merges, as long as the data/merge is correct. You can always request and most people will be happy to work with you.
I won't speak for others, but when I lock profiles, it's because I've gotten them to a very good state, with correct children and parents or nearly there, and they are profiles that people tend to either merge erroneously (multiple generations with the same names) or are constantly changing data to not comply with Geni naming standards. I'm counting on collaborators to improve the profiles and it's great when people come with new information.