Error when pressing add button for a Common Ancestor

Do you mean this John Howland, "Mayflower" Passenger?
You will also probably be interested in the Mayflower project over at: http://www.geni.com/projects/-Mayflower-Passengers-Colonial-America...

If you contact Curator Erica Howton, who started that project, she will be happy to get your tree connected into it (probably through a much more recent profile).

You can't just connect to any profile. That would create complete chaos. You have to Collaborate with someone who manages that profile.

You CAN then connect your profile to that person's profiles. You do this by looking at the profile in the Tree, and under the "More" options press Move. This will put a copy of the profile in a menu on the right side. You then open the other tree, and drag-and-drop the first profile from the menu onto the profile you want to connect to, and choose the appropriate connection type, in the menu that opens.

I have multiple lineages descending from John Howland, and two from his brother Arthur. Your Geni.com does not seem to recognize intermarriages between cousins. I also have eleven other Mayflower lineages that were all mixed up incorrectly when someone tried to merge them. Now I cannot figure out how to fix my lineages. Perhaps I should withdraw altogether from this project. At this point I cannot recomment Geni.com If there is a way to fix it- it is not intuitively obvious.

There is a way to fix the tree. That's by posting the public profile URL of the profile in question to the public discussion, "ATTENTION Curators, please assist", which is here:

http://www.geni.com/discussions/80793

Again, we need the profile URL to help (50 million profiles in geni). You can copy and paste it from the address bar of the public profile to the discussion.

Hope this helps.

@ Private User - your best bet is to do what Erica suggested and make sure that your early America profiles are curated and are master profiles. I have gotten a lot of help to clean up the trees of the families I belong to who came on the Mayflower or in the Great Migration. I have done this two ways - posting in the ATTENTION Curators thread or in the Mayflower Passengers thread or creating a project for some of these early families.