Held means that those seats can't be sold or used by non revs. They are held for relief flight crews or aircraft limitations for weight or performance factors.
Groups are just that. A group of pax travelling together, such as a high school club, church group, etc. For non revving purposes, a large group will almost always show up for the flight. On some legs, up to 15% of the pax won't show, LAS is an example, so if the PBT is tight, you might just get on.