I have a simple policy when it comes to cost-sharing- eachone pays for himself and where it is possible to share costs (taxi, accommodation etc), we share. Please note that I do not intend to make any money out of this, hence there will be no fee component or deposits to be made.
I am an avid traveller but due to the nature of my work I seldom travel on weekends and hence miss out most of the trips that my friends/travel buddies plan. Hence I decided to start a group where people with same constraint can come together and find co-travellers. Though one can always seek co-travellers for weekends as well.
There is this sentence I often use- There is a difference in being a tourist and a traveller :-)