If it’s dark and/or a white out and you have a large group of people to keep together, here’s a couple of techniques. They might sound a little cheesy if you’re reading this on a bright sunny day, but each can bring peace of mind to the team, and some psychological comfort that everyone is part of the group and not isolated.
(Of course, the leader can remind people to take breaks when everyone in the group stops and not to randomly stop whenever they feel like it, which can easily lead to getting lost in the dark.)
1 - Assign everyone a buddy. After a break and when you’re ready to get going again, the leader calls out “buddy check”. Everyone on the team looks around and hopefully finds their buddy. If they see them, they don’t say anything. But if their buddy isn’t there, they give a shout to indicate someone is missing.
2 - Count off. Everyone on the team gets a consecutive number, and they shut out the number one at a time to make sure everyone is accounted for. Make one person the leader, “one”, so when they call “one”, everyone answers in order: “two”, “three”, etc.
This tip is partially from the excellent book "1001 Climbing Tips" by Andy Kirkpatrick