A number of published adventures mention rolling a "group stealth check" and other group checks where, theoretically, a proportion of the players try to beat the DC.
Are there formal rules for this, and if so, where?
Rules Compendium page 128 states that group checks are almost always against an easy DC and if at least half the group succeeds the whole group succeeds. Otherwise the group fails.
Group Skill Checks
Sometimes a skill challenge calls for a group skill check. When the party is climbing a cliff, everyone needs to roll an Athletics check to climb. In this case, allow one character to be the lead climber. This character makes the actual check to gain a success or failure. The others make checks to help the lead character, in effect aiding that character, but their checks provide neither a success nor a failure toward resolving the challenge. Each ally that gets a result of 10 or higher provides a +2 bonus to the lead character’s check (to a maximum bonus of +8).