Many individuals prefer to work on "Project" tasks that have a defined objective with specific tasks and timescales.
This means the participants can provide their skills and their time for a defined period, not open ended which can make them feel like as though they have an obligation.
The use of the Trading system built into Agoria as well as accounting for hours and effort can also package up the skills needed for specific projects.
It also resolves one of the conflicts for a volunteer, they can specifically state how much time they can spend on a project.
I have worked with some projects which take place on Saturday mornings in the Spring. I normally participate in two of the four that take place for three hours each. In this way I can choose my Saturday and location specify three hours and then I can leave them to it, or I can take the late shift and leave when they finish.
For the organisers it takes a little more planning but by the nature of projects the job is planned to the time available.
These projects are normally spin off's from another community group that is requesting the help for a short time. As a consequence these small projects enable members from other groups to meet and compare notes.