In some cases, when adventuring, a party may need to move silently. In some cases, a single character may need to make a check - such as when one character is leading the party in the wilderness. In other cases, where it is unreasonable for only a single partty member to make a check, a group check is called for instead.
When a group check is made, if at least half the party succeeds, the group as a whole succeeds. If more than half the party fails, the group as a whole also fails.
For example, when the party needs to move stealthily, a group check may be used, with the more successful party members, aiding the less stealthy.
In some cases, a Group check may still end up as a Pass or Fail, in some cases where a party member in full plate fails their Stealth check terribly, this can have a full adverse affect on the party's stealth capability.
In cases where a party member is unable to succeed on a check, but could have reasonably passed with assistance from party members, if the aid is roleplayed well enough, the DM may allow it.