Guild skills passively give more HP, MP, Vitality and more. The acquired skills are also retained if you leave the guild and join a less developed guild.
First you have to develop the skills so that every member can learn them.
At the moment the max lvl of skills in guild lvl 5 is 30.
Open the guild window and click on Guild Skills.
In the upper right corner you can see the progress of the skills.
On the left you can use textbooks to speed up the development.
At the bottom you can find the buttons to start and stop the upgrade, and also to learn a selected skill.
To learn a skill, the player needs some gold and guild fees (which you get by doing the guild tasks)