You can create a page for admin from where he can add users something like
aspx
<tr>
<td>
Username:</td>
<td>
<asp:TextBox ID="txtUserName" runat="server"></asp:TextBox>
</td>
</tr>
<tr>
<td>
Password:</td>
<td>
<asp:TextBox ID="txtUserPass" runat="server" TextMode="Password"></asp:TextBox>
</td>
</tr>
<tr>
<td valign=top>
Roles:</td>
<td>
<asp:CheckBoxList ID="cblRoles" runat="server">
<asp:ListItem>Admin</asp:ListItem>
<asp:ListItem>Role 1</asp:ListItem>
<asp:ListItem>Role 2</asp:ListItem>
</asp:CheckBoxList></td>
</tr>
<tr>
<td>
</td>
<td>
<asp:Button ID="btnSave" runat="server" Text="Save" onclick="btnSave_Click" />
</td>
</tr>
aspx.cs
protected void btnSave_Click(object sender, EventArgs e)
{
MembershipCreateStatus createStatus;
MembershipUser newUser = Membership.CreateUser(txtUserName.Text, txtUserPass.Text, null, null, null, true, out createStatus);
if (newUser != null)
{
foreach (ListItem li in cblRoles.Items)
{
if (li.Selected)
{
Roles.AddUserToRole(txtUserName.Text, li.Value.ToString());
}
}
}
}