As for now there is no way to `connect` members trees what you can do is contact the webmasters and ask for them to share the information with you.
We are working on the feature to merge trees and I hope it will be available very soon!
If you have an online tree, members of your site can see and make changes. As for quests although they can see most of the information on the site if you choose, they do not have the privilege to edit your tree. This is so in order to protect your privacy. If you want guests to edit your tree you need to invite them to your tree as members!
If you have built your tree on FTB and then published it then no one else but you can edit the tree. You need to make all your changes on FTB and then republish.
So now if you have an FTB tree and want to convert it to a tree which members can edit you can have an online tree by completing the following steps.
Open your family tree in Family Tree Builder.
Go to File > Export Gedcom.
Now go to your family site.
Click on Family Tree > Import Gedcom.
Browse your computer to find the gedcom you just created.
Click upload and that's it!
Now you have a family tree that you can work on online. **Keep in mind that the Import Gedcom feature on family sites will not include photos. You'll have to include them separately.
If you need help with any of this, please let me know.