Where Parents Actually Talk
The official channels are great for official things. These are where you ask “does anyone have a size 6 polo” at 9pm the night before.
The two groups worth joining
ISO means “in search of.” If you need something specific and nobody's currently selling it, just post “ISO size 7 khaki shorts” in the Marketplace group and wait. Somebody almost always has one in a closet. It's the single most useful thing to know about that group, and nobody tells you.
How to actually use them
Join both, then mute them. They're most valuable as a searchable archive, since someone has almost certainly already asked your question, and least valuable as another notification stream competing for your attention.
Before buying uniforms new, search the Marketplace group for the item first, then post an ISO if you strike out. See the Supplies & Uniforms page for where to buy the things you can't find secondhand.