Detailed Idea Description
Set a goal for players and give a prize for achieving the goal
For example, when 40 people are reached in one server category SURVIVAL, everyone gets a certain prize (key or other drop, etc.).
Make such a PARTY every 24h (So that prizes are given every 24h) If the number of people is higher then raise this requirement to 60-70.
The same can be done with the total player count across all servers.