In WoW classic the guild name is only shown on the nameplate, not in the tooltip.
This addon adds a line with the characters guild name and rank to the tooltip that is shown when you place the mouse over the character.
Here is a sample of how the tooltip look with the rank (Member) and the guild name (Crazy Raiders):
The idea came from Gazmik Fizzwidget's Guildentipper, but the old API calls caused too much trouble. This addon is made to be very simple with clean API calls that should not cause trouble.