You can establish a Guild after lvl 30, just search for the NPC Guild Manager to create one.
Input the Guild's name (maximum 12 characters). If the name is accepted, you'll receive a message saying "Application for Guild creation has been Successful".
You must invite 10 people in 24h or your newly created guild will be disbanded.
Click the Guild Menu Icon to see your new guild info and member list.
You can invite a player to join your guild by directly left clicking on them twice and choosing the Guild Invite option, or by going to the Guild Menu and selecting "Guild Invite".