[RPG] Can Resilient Sphere contain multiple creatures

1. If your target succeeds on the Ref save, the sphere doesn't form in the first place: "Reflex negates", thus the effect (1-foot-diameter/level sphere centered around a creature) cannot be triggered, thus the sphere cannot form.
2. Probably, and probably. The spell description is a bit vague, but it seems entirely reasonable that others could be trapped within the sphere (the description suggests this is the case: "The sphere cannot be moved ... by the struggles of those within.") and that those others would get a ref save to dive out of the way.

I advise conferring with the GM about how they would adjudicate such a sphere catching multiple creatures before spending too many resources on it (which may require casting the spell in a mock combat situation with your party).

If you're the GM, decide how it works and be consistent. It's fine if other spells (or other force-effects, even Forcecage) work differently, but make sure that Resilient Sphere always works the same (barring specific situational modifiers, of course).

Were I GMing, I would give non-targets who would be caught a ref save to dive out of the way, in the direction of their choosing (using up movement from their next action); an acrobatics check (or saving by 5+/hitting a nat-20 on the check) would let them end the movement standing, else they'd be prone. Large (or larger) creatures who are partially in the space would get the ref save to choose whether to end up inside or outside of the sphere; failure would put them on whichever side the caster wants them to be on.

The important part of that attack option is this:

And the water weird can't constrict another target

This same terminology is used of creatures like the Tarrasque who have the 'Bite and Swallow' feature or a Kuo-Toa Whip's Pincer Staff.

This means that the Weird cannot Constrict any target other than the one that it is presently constricting. So it can repeat this attack against the same target every round, in addition to the fact that you are being held underwater. Most likely, it will kill you by crushing rather than killing you by drowning.

In fact, the lore comments on it even makes it clear that it is trying to crush you.

[...] using its coils to crush any creature other than its summoner [...]

One other point of note when fighting a Water Weird: It has a 10' reach, and can fight just fine in deep water. The party may have a difficult time reaching it if it is sitting in water too deep to stand in. This can make it a little extra challenging as well if the party has to jump in and try to swim in order to fight it in melee.