If you can't host it on your own server... then change to gitlab you can make private repositories as contrary to github without paying but you should pay for being able to make commits as a group or agency. The most important thing is that you will save a lot of time. Probably hours every day, whenever you're uploading your code (since the moment you stop coding and start looking for the HD) take times and don't stop till you merge every change with the code you receive and so, then do some maths to take account the co-workers time and then make more maths to calculate based on their salary how much money does it cost the company to make things the way they're doing it.... I can bet you that seeing the loss in money they will change immediately.
If all of that does not work you could make a localhost gitlab hosting, I think this should work in a way easier and cheaper sense, but you won't be able to pull anything if the computer isn't on and won't be able if you're not connected to same wifi or lan.