You can use SPUser class to retrieve all users for the site. The details
can be found from Sharepoin SDK. I am attaching portion of this which
help you in this regard.
SPUser Class
The SPUser class represents a user in Microsoft Windows SharePoint
Services.
System.Object
Microsoft.SharePoint.SPMember
Microsoft.SharePoint.SPUser
[Visual Basic]Public Class SPUser
Inherits Microsoft.SharePoint.SPMember
[C#]public class SPUser : Microsoft.SharePoint.SPMember
Public Methods
The following table shows the public methods of the SPUser class and a
brief description of each.
Name Description
ToString Returns the user name of the user.
Update Updates the database with changes made to the properties of the
user object.
Public Properties
The following table shows the public properties of the SPUser class, the
data type of each property, and a brief description of each.
Name Data Type Description
Alerts Microsoft.SharePoint.SPAlertCollection Gets the collection of
alerts for the user.
Email String Gets or sets the e-mail address of the user.
Groups Microsoft.SharePoint.SPGroupCollection Gets the collection of
cross-site groups of which the user is a member.
ID Int32 Gets the member ID for the user.
IsDomainGroup Boolean Gets a Boolean value that indicates whether the
user is a domain group.
IsSiteAdmin Boolean Gets or sets a Boolean value that specifies whether
the user is a site collection administrator.
LoginName String Gets the user name of the user.
Name String Gets or sets the display name of the user.
Notes String Gets or sets notes for the user.
OwnedGroups Microsoft.SharePoint.SPGroupCollection Gets the collection
of cross-site groups of which the user is an owner.
ParentWeb Microsoft.SharePoint.SPWeb Gets the parent site for the user.
Roles Microsoft.SharePoint.SPRoleCollection Gets the collection of site
groups of which the user is a member.
Sid String Gets the unique security ID for the network account of the
user.
Xml String Gets the XML representation of the user object.
Thread Safety
Any public static (Shared in Visual Basic) members of this type are safe
for multithreaded operations. Instance members are not guaranteed to be
thread safe.
Remarks
Use the AllUsers or SiteUsers property of the SPWeb class, the
GetAllAuthenticatedUsers method of the SPUtility class, or the
GetUniqueUsers method of the SPAlertCollection class, to return all the
users of a site, all the users in the site collection, all the
authenticated users of a site, or a list of users for a collection of
alerts. Otherwise, use the Users property of the SPGroup, SPRole, or
SPWeb class to return the users in a cross-site group, site group, or
site. Use an indexer to return a single user from the collection. For
example, if the collection is assigned to a variable named myUsers, use
myUsers[index] in C#, or myUsers(index) in Visual Basic .NET, where
index is either the index number of the user in the collection or the
user name of the user.
Every user has a unique member ID, has the permissions associated with
that membership, and can be represented by an SPMember object. The
following example assigns a user to an SPMember object:
SPMember myMember = site.Users["User_Name"];
For general information about users and security, see Security, Users,
and Groups Overview.
Example
The following code example modifies the e-mail address, display name,
and notes for a specified user object.
[Visual Basic .NET]Dim siteCollection As SPSite =
SPControl.GetContextSite(Context)
Dim site As SPWeb = siteCollection.AllWebs("Site_Name")
Dim user As SPUser = site.AllUsers("User_Name")
user.Email = "E-mail_Address"
user.Name = "Display_Name"
user.Notes = "User_Notes"
user.Update()
[C#]SPSite siteCollection = SPControl.GetContextSite(Context);
SPWeb site = siteCollection.AllWebs["Site_Name"];
SPUser user = site.AllUsers["User_Name"];
user.Email = "E-mail_Address";
user.Name = "Display_Name";
user.Notes = "User_Notes";
user.Update();
This example requires using directives (Imports in Visual Basic) for
both the Microsoft.SharePoint and Microsoft.SharePoint.WebControls
namespaces.
Requirements
Namespace: Microsoft.SharePoint
Platforms: Microsoft Windows Server 2003
Assembly: Windows SharePoint Services (in Microsoft.SharePoint.dll)
Security: Code Access Security