Helpers & Point Persons
If you want outside help on a check to improve the result, the GM will decide whether it makes sense if your helpers can actually aid you in the task. In cases where more helpers means a better result (such as lifting a heavy object under duress), or a more experienced helper is necessary to pull it off (they have a skill you don't), the GM may grant advantage on the roll.
Sometimes, more cooks in the kitchen just won't help. If you already have advantage from group action, adding more helpers to the mix can't improve the roll, unless adding more helpers in a countable way makes sense. For example, if you're adding more helpers to move a heavy log, the GM may confer a +1 modifier for each helper, provided their Mighty is equal to or greater than yours.
In either case, select a PC from the party to act as a point person, and that PC rolls. Their success or failure represents success or failure for all.
Piggybacking on a Roll
Sometimes you're making a check on behalf of others (such as establishing a stealth score for a group of people you are helping to hide in shadows). In that case, the GM may impose disadvantage on the roll. If the piggybackers are countable and the exact number of them matters, the GM may impose a -1 modifier for each extra piggybacker instead.
Finally, the GM may grant a check to compensate for the penalty applied, if you take precautions before attempting the roll.