View members in a group:
Get-ADGroupMember -identity groupname | sort name | select-object -expand Name
Same as above, but does a recursive search through all sub-groups:
Get-ADGroupMember -identity groupname -Recursive | sort name | select-object -expand Name
View groups a single person is in:
Get-ADPrincipalGroupMembership username | select name,distinguishedName | sort distinguishedName