I co-invested a domain name with a domainer friend in 2009.
He found that domain name first but lack of funds, so he talked with me and we paid together (50/50, and the total price is $7,000)
We agreed to hold it as a long term investment for at least 12 months before we actively selling it to end-users. However, after around 2 months, he asked me if I would like to buy his 50% because he need some quick cash to buy something else. So I bought the domain back. I sold it later for $68,000 in 2014. Not a premium deal though.
Actually I feel bad about it. My advice, don't co-invest in domain names if you can afford that domain name alone. Or you should try domain loans.