[This guide is for Rift.]
If you want to port to where your guildmate is:
During events the guild often will ask you to be ready to be summoned to a location. This is how you prepare for that, so you won't miss the short time you have to port once your guildmate uses the skill to summon you.
Go to Sanctum. You will need to go to the tavern on the east side, outdoors (loc 7611, 3104). Find the npc Gilbert Hilberto (Guild Charter Magistrate) inside the tavern. He is by the fireplace. From him you will need to buy a Rally Scroll for 10g. I recommend you buy several. (Defiants go to Meridian and enter the middle archway, then when you get into the room, turn around and there is a guy behind a desk.)
You should take this item and drag it to your action bar just like you would a potion. Now wait for the order to port. You will see in guild chat the message (as an example):
Foghladha calls you to her side in Wherever!
Now, click on the Rally Scroll. There is no time to hesitate, you need to do it asap. The time in which you can port after you see this message is very short.
Please do not expect your guildmates to keep putting down banners because you missed the port over and over. It costs 50g for them to port you, every time. There is also a cooldown of one hour before that person can summon you again. So please be ready for when the order is given. If you are in vent you can be sure not to miss the message.
If you want to port your guildmates to where you are:
You will need to prepare ahead of time. Go to Sanctum. You will need to go to the tavern on the east side, outdoors (loc 7611, 3104). Find the npc Gilbert Hilberto (Guild Charter Magistrate) inside the tavern. He is by the fireplace. From him you will need to buy a Rally Banner for 50g. I recommend buying two or three if you can afford it. (Defiants go to Meridian and enter the middle archway, then when you get into the room, turn around and there is a guy behind a desk.)
Next you will want to press P to open your Abilities window. Click on Guild Perks. Drag the Summon Guild Rally Banner icon to your action bar just like you would a skill, because that's exactly what it is. The difference is, in order to use this skill, it requires a consumable item, the Rally Banner, which you have just bought.
Next announce in vent and in guild chat that you will be summoning your guildmates and for what reason, such as a specific event, and give them some time to prepare. Repeat this several times. This will give people time to get to Sanctum and buy their Rally Scrolls. When you are reasonably sure that everyone is prepared, give them even more time!
Take this extra time to make sure you are not summoning low level players into a dangerous area where they will be immediately aggroed. Scout around for a safe place.
Now that you are really sure that everyone has their Rally Scrolls, and that you are in a good place for them to appear, announce in vent and guild chat that you are about to summon them.
Click on the Summon Guild Rally Banner skill icon. Do not move away from the banner until everyone has ported, otherwise you will cause it to despawn too soon. Also do not click on the banner as this will also cause it to poof.
That's all there is to it!
Some people will inevitably miss the port. Maybe someone else in the event can summon them?