You can nest either a gridview or repeater inside your Gridview and use the relationship to populate the data by setting the datasource to the relationship:
<asp:EntityDataSource ID="eds" runat="server"
ContextTypeName="people"
EnableFlattening="False"
EntitySetName="people"
Include="users" >
</asp:EntityDataSource>
<asp:GridView ID="gv" runat="server" AutoGenerateColumns="False" DataKeyNames="PeopleId" DataSourceID="eds">
<Columns>
<asp:BoundField DataField="FirstName" HeaderText="First name" />
<asp:BoundField DataField="LastName" HeaderText="Last name" />
<asp:TemplateField HeaderText="People">
<ItemTemplate>
<asp:Repeater ID="rptUsers" runat="server" DataSource='<%# Eval("users") %>'>
<ItemTemplate>
<%# Eval("username") %>
</ItemTemplate>
</asp:Repeater>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
This assumes you have a relationship setup in your data model, eg:
public class person(){
public int PersonId{get;set;}
public string FirstName{ get;set;}
public string LastName{ get;set;}
pubic virtual ICollection<user> users { get; set; }
}
public class user(){
public int UserId{ get;set;}
public string username{ get;set; }
}