Guild members can help each other level up their heroes.
To request donations, simply tap the Fuse Request button and select which hero you wish to upgrade. You can only request fuse for one hero at a time, and only so often. Your guild members can then donate their heroes as a sacrifice to level up your hero. You can collect your request after it's full or when the timer runs out. You can make a fuse request again after the time shown on the Fuse Request button resets.
If your guild mate makes a Fuse Request, it will be visible to everyone in the guild until its full or time is up. Be quick to check your Guild tab. Requests fill up quickly, and you might miss them.
You can donate one hero at a time, and you can make six donations per day. Donating will give you the same amount of Gold as selling the hero, plus some Guild Points. Collect enough Guild Points, and you can use them in the Guild Shop under the Shop tab.