How do people collaborate on maps

The first thing you would need to do would be verify your part of the level, then go the level selection screen in the "create" tab and select your level.

Once you've selected the level you want to collab on, click the red box with an arrow coming out of it.

This will lead you to a screen that has an option to edit the description and request the rating for your level, among other things, but what we're interested in right now is the little gear in the top right corner.

Clicking this gear will allow you to make your level available for other to download and edit, but, assuming you only want one person to have access to this level, you'll want to add a passcode and PM it to the person whom you want to have access to your level. you can do this by clicking the "requires password" box, and then clicking the "enter password" box.

After you click it, you'll be presented with a keypad, enter the passcode you want on this keypad, and then click the "OK" button. After all of this, you can publish your level, and your friend can copy it, so that they may add their part (or verify) to the level. They can also repeat this process if you want to have more people involved. Hope this helps!

Winterblade's answer is a good one, but there's another way to collaborate now — one that probably appeared only in a recent update.

After you check off "allow copy", another checkbox appears: "Unlisted". If this box is checked when the level is published, the level won't appear on any searches anywhere unless the level is searched with its Level ID (The Level ID is the series of numbers on the level's page as shown in Winterblade's post's first picture. Before the level is published, it will say, "ID: na”).

Assuming both you and your collaborator have Geometry Dash accounts, you can go to Opening screen > Level Editor > Create > [Name of your level], write down the ID, and message it to your collaborator (Opening screen > [Blue profile button] > [Third button in bottom row with two heads] > [Collaborator's name] > [Blue envelope] > [Type message and subject] > [Send]). They then can find the level, copy it, and build their own part, publishing it, too as unlisted. After they message you the ID of the level with their additional part, you can delete your old version or remove it from the server (Opening screen > Level Editor > Search > [Type in Level's ID] > Get It/View > [Second button on left column, gray box with red X] > Yes), copy their version, add your stuff to it, and republish it as unlisted.

Keep doing this until the level is finished, then publish it (without unlisted on!).