It not just the domain, but what you are doing with the domain. Is it a money making business, a non-profit information or fund-raising venture, perhaps you want to keep a trade name for software that you are all developing or investing in.
If so, this all needs to be addressed from a legal and even tax standpoint. If it is something really simple with hardly any money involved at this point, then you might get away with one of the excellent Nolo Press partnership books to guide you. But you need to have a specific agreement that addresses what to do when you all do not agree on what to do. What are the criteria for calling a vote - how much lead time? Who gets to vote? What if one of the parties is serving in the military and can't be reached? How many votes are considered sufficient to pass the proposal? etc. etc.
The best way I can think of is to appoint an unbiased third party like an attorney to control the domain. Maybe even in a formal trust arrangment. Where nothing can be changed unless there is proof made to the attorney of a sufficent number of "votes" to institue a change in whatever it is you want to do.
There's a thousand different ways to set this up depending upon how much you want to spend.
You may delay, but time will not. --- Benjamin Franklin