Hi I am UK based and I'm recently forming a team to develop a game. I'm the lead developer and will be bringing on ~5 other artists to create content such as models, textures and music.
For the moment, I have not formed an official company, to avoid the time sink of all the paperwork, contract writing and so on. But without agreements/contracts/licences in place as I understand it, an artist on my team could pull out at any moment and reserve the right to take all their content with them.
To prevent this, I would like to have them upload their content to my website when they are done with it, and to include a licence in that directory that says something along the lines of
"By uploading your content files to this directory, you agree to transfer ownership of your files to the lead developer Alex Lynch, in exchange for payment from the revenue of the game".
I hope this would be enough, until I form a real company and get signed contracts from them.