I have a Master Subscription and although Content Sharing is active in my campaign, other players cannot see the options in character builder. 

Try toggling Content Sharing on and off. Also, the other players' characters must be in the campaign to have the character builder options. If they are, they may need to select something temporarily for a race/class, then go back to the race/class page in order to see the shared options.

(And of course, someone in the group needs to own some content for it to be shared! I assume someone does, just covering bases...)

Quote from naruhoodie >>

(And of course, someone in the group needs to own some content for it to be shared! I assume someone does, just covering bases...)

 
To expand on that, subscription does not unlock any content whatsoever.

In addition, if the source you are trying to share are Eberron, Wildemount, Ravnica, or Theros, additional toggles need to be flipped on the Home tab for each character, “Eberron Content,” “Critical Role Content,” and “Magic: the Gathering Content” respectively. The toggled need to be red (to the right).

Thanks guys.  I knew that stuff and have all of the sourcebooks as well which is what was baffling me. but I solved the problem.  Players need to make sure their "Entitlements are Synced" in each of their individual accounts.  Kind of a pain in the ass, but an easy fix.

How do I do this?

Sync your entitlements? Go here, and check the section right near the top labeled 'Account Entitlements.' If it says they're in sync, you don't need to do anything. If they're not in sync, it will say so and have a link or a button to re-sync them. If you meant something else, please clarify.