/
Each player can place a limited amount of Eyes of Ender per End Portal.
Players can remove Eyes of Ender from an End Portal, but only those that they've placed.
Even if the End Portal is filled, all players need to be present in the portal room before it will open.
Even if one contributor is missing, the End Portal will not open.
Once the End Portal is filled and all contributors are present, the End Portal will open and remain open as long as all contributors remain present.
An End Portal will also remain open as long as one of its contributors is alive in the End (i.e. currently fighting the dragon).

Softly restrict the End with vanilla-ish gameplay mechanics that make sure your SMP's dragon fight is always a group activity!