You can assign volunteers to timeslots within an event by choosing from two quick methods.
Allocate volunteers using the plus icon
Click the plus icon next to the volunteer's name in the Invitations section.
Click Allocate timeslots.
Select the timeslot you want.
Click Update.
Assign volunteers using the person icon
Click the person icon to the left of the volunteer’s name.
Select the timeslot you want.
Click Update.