This is a job for a linking table.
you would have a volunteers (Users) table.
a tasks table
and a userstasks table.
the userstasks table would have the following structure
userstaskID - Primary Key
userstaskUserID - relates to the useers.UserID
userstaskTaskID - relates to the tasks.taskID column
the user would log in, there ID would be stored in a session, they would select the tasks, then use a multiple insert behavior to insert to the userstaks table the userID session and TaskID's that where selected.
this is similar to the items -> Genres concept from the Manage relational table tutorial: