The standard group check “the best skill & characteristic” doesn’t apply well for Stealth. RAW says each PC rolls individually, and may pool extra successes—the good ones help the noisy ones. But that can take a long time for large groups, and if you have them leading NPC minion groups it doesn’t logically work for the minion group to roll once, since it’s harder for large groups to hide.
So I have the following house rule:
Large group at Short range: Take group check (best rank + best Agility) vs opposed Perception . Add Setback for each N/PC with no ranks in stealth, and upgrade difficulty once for each N/PC with Agility of 1, and add Boost for each other N/PC with the same “best” Stealth rank. Note: Minion groups count individually, since larger groups are hard to hide.
- Modifiers: +1 Ability die for Med Range; +2 for Long, +3 Extreme. Upgrade if out of Ability dice. [edited per below] See limits of dice via Foolproof rule below.
- Downgrade difficulty if perceiving target must guard 360 degrees, or outside their ocular field.
- Consider if target using scanners.
- Standard environmental difficulty modifiers also apply for crowds, concealment, etc.