Create a Group:
CP -> Forums -> Group Management -> "Add New Group" tab at the bottom
Set Group to have CP Access:
On the "Group Management" page: Edit Permissions -> CP Permissions (for this group)
Set the permission: "Can edit users"
Assign User to Group:
Find user in User Management and add them to the group.
Any user in the group can edit users in the control panel, they'll have an "Edit User" link when viewing profiles and have a link displayed for control panel access (though they're only allowed to use member management).