A volunteer cannot see their schedule but they say they've created  a Timecounts account and have logged in. 

Make sure they are logged into Timecounts using the same email you have for them in your community directory. They need to match in order to give them access to your hub. They can edit their email in their user settings by clicking bottom left corner and going to 'Profile'. You will also want to share the link directly to your hub so they don't get lost and is able to find your opportunities right away.

A volunteer signed up for a shift and then cancelled it in the hub. The shift disappeared from the volunteer's My Commitments tab, but if they click back into My Activities for the assignment they do not see the shift again in the schedule. Did they do something wrong?

If a volunteer cancels a shift they were previously committed to, we do not put that shift back into the schedule for them again (it will appear for other volunteers). Instead, they will find it has moved under their Commitments filter for 'Cancelled'. If you look at the list as a new volunteer, that shift will appear for them in the list.