Is there any way to share compendium with more than 15 people?

I'm staffing a D&D server that formats itself like a guild, and we have a *lot* of players. ~40 right now in the roll20. I saw that you can only share with 15 people with Pro subscription, but the server benefits newer players and newer DMs and I want them to benefit from the added help the compendium offers to minimize the amount they need to learn right off the bat.

No, my suggestion is that you use multiple games. Cycle new players through a "creation game", then migrate their character to the "active game".  If you have approximately 40 players you could also use multiple games to give each group of 15 access to the external compendium. It would take 3 games to do that.