Display a list of users with many properties (admin section on a website) [duplicate]

Usually, when I start to create a list of things in a web application, I start by displaying a table

╭───╥────────────┬─────────────╮
│   ║  Name      │   Email     │
╞═══╬════════════╪═════════════╡
│ 1 ║ First      │ a@b.ux      │
│ 2 ║ Lorem      │ ipsum       │
└───╨────────────┴─────────────┘

Then the web app grows and I need to display more things:

╭───╥────────────┬─────────────┬─────────────┬─────────────╮
│   ║  Name      │   Email     │   etc.      │   etc2.     │
╞═══╬════════════╪═════════════╪═════════════╪═════════════╡
│ 1 ║ First      │ a@b.ux      │Lorem        │ ipsum       │
│ 2 ║ Lorem      │ ipsum       │Lorem        │ ipsum       │
└───╨────────────┴─────────────┴─────────────┴─────────────┘

Then it start to become unreadable, because there are too many columns.

Assuming all of these fields/properties are important (I mean, they have to be displayed in the list), what can I do to display all this information ?

Example of properties: name, firstname, email, group, departement, role, last connected at, action, ...

DISCLAIMER: I'm not a UX designer. And it may be my first question here: I can edit if I misunderstood some rules about UX questions.