So the other partymembers have no inventory. Pushing the inventory button, n, They are shown in a screen like any monster would.
If this is during the initial part of the game with Lae'Zel and Shadowheart on the Nautiloid, that's intentional and expected behaviour.
If you still have inventory issues after reaching the mainland, I'd suggest checking that
\Root\AppData\Local\Larian Studios\Baldur's Gate 3\Mods directory is empty, then make a new player profile in the game's main menu and start a new campaign to see if that resolves it. (Mods are profile-bound, and making a new profile sometimes helps with reverting to a blank slate in terms of what the game tries to load / expects to load)
