v.
1) ( D ; intr. ) to ~ as (to ~ as a volunteer)
2) ( D ; intr. , tr. ) to ~ for (she signed up for an evening course; to sign smb. up for a course)
3) ( D ; intr. , tr. ) to ~ with (they signed up with the volunteers)
4) ( E ) they signed up to serve as volunteers
5) ( H ) they signed us up to serve as volunteers