Baldur's Gate 3

Husband and I have been playing for a few weeks (with early access) and that's how it was for us. If you manage to pull off a surprise attack while still in stealth, you can get more as long as your character(s) are not seen. However if the enemy spots just one (or more) of your characters, let's say the "surprise attacker", they will start the next round without those characters not seen or engaged in the attack until you bring them in sight or in combat. Essentially, you can attempt to use that as a surprise attack for a second/third character if they attack from outside the visual range of the enemy. Then they will get placed into the initiative queue. At least that is what we've experienced.

[Potential Spoilers] Best example is outside the cathedral with Gimblebock, the mage, the archer and the other guy. If we sneak up behind the archer, we can kill him without the others spotting us. Then we get our furthest ranged casts and dual hit the mage at the same time, killing her. At that point the other two downstairs realize there's a fight and we force them to come upstairs to fight us. Still may have to walk the non-caster party members to actively join the combat queue.

Before you end turn of your suprise attacker, use that time to bring everyone else into combat, then end your turn. We learned that the hard way, fighting a group of mobs when Shadowheart or some other companion is still just sitting outside.