I want to add a 5TB HD (NTFS) to my time capsule and have access to it from both my Mac and my PC's
Here is a link to Apple's instructions on how to mount the drives on both sides if they don't seem to show up. How to mount a Time Capsule HD

The only disk formats that the Time Capsules support are HFS (Macs default), FAT16, and FAT32. Supported HD for Time Capsule. Since you have a large HD you cannot use FAT16 as it is made for smaller HD's (&lt;2GB).

If you would like to use Apple's Default (HFS) you will have to install software on the PC in order to write to the drive. Paragon-software has the tools needed to use HFS on windows computers

If you use FAT32 both your windows and Mac will be able to read/write from the HD.