Let user's enter values?

I have an image and I want users to be able to enter data about the image. For instance a field that says “Who” and users can enter data into that field by a simple contact form. The data they enter will be “added” to data that is already in that field. Such as user 1 enters Andy and user 2 enters Rip. When I list the values for the Who field I should see “Andy” and “Rip”. Basically an array. I understand how I want it to work but I can’t figure out what method I use for the users to be able to enter such data.

Any suggestions or can you point me towards a plugin that might currently allow this?

Any help is appreciated. Thanks!