There are several ways to collaborate on an image between users.
Allow Anyone to Edit
The Allow Anyone to Edit (AAE) feature makes an image editable to anyone who can view it. The pencil edit icon, usually reserved for the creator, is publicly visible and other can add tags, edit existing tags, and delete tags.
To enable AAE, click Settings beneath the image while in edit mode. In the settings click the AAE check box.
Log In As Group
By signing in as your group the image is in the group's account and can be accessed and edited by any group member. Though it is slightly more difficult to access the image, this method has the bonus of only being editable to group members.
In the upper right menu, use the drop down to log in as your group.
After logging in as the group you can go to the create tab and upload and tag just like you normally would. After you have completed your edits, go back to the upper right menu and use the log out icon to exit your group and return to your personal profile. Other group members can also log in to the group the same way and add edits or their own images.