How can my Windows XP computer join my Windows 7 laptop's HomeGroup?
See this Microsoft article :
In short, only Win7 computers can fully join in the HomeGroup, but the network is still usable from XP in the classical way, through "My Network Places" -> "View workgroup computers", double-click the computer you want to access and enter the credentials when asked.
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It can't. Homegroups are only supported in Windows 7. They can, however share files and printers using good old networking.