Ability to flag a Volley as "checked in" for the Shift that they signed up for.

Ability to flag a Volley as "checked in" for the Shift that they signed up for.
Q) Can one use "hook form alter" for this feature? What about the serialized data?

Once Volleys have "checked in", then need the following report onsite:
- Which shifts are full -vs- which shifts need filling