HttpCode

The following code will create a site based on a template, then it will add a new contributor group:

//create the subsite .. subSite is an SPWeb object
SPWeb createdWeb = subSite.Webs.Add("Url", "Title", "Project Subsite", 1033, "Template Name", true, false);

createdWeb.BreakRoleInheritance(true);

SPMember member = createdWeb.Users[createdWeb.Author.LoginName];
//create the user groups ...

createdWeb.SiteGroups.Add(createdWeb.Title + " Contributors", member, createdWeb.Author, "Contributors to the site");
SPGroup newContribGroup = createdWeb.SiteGroups[createdWeb.Title + " Contributors"];

SPRoleDefinition contribRole = createdWeb.RoleDefinitions.GetByType(SPRoleType.Contributor);
SPRoleAssignment contribRoleAssignment = new SPRoleAssignment(newContribGroup);
contribRoleAssignment.RoleDefinitionBindings.Add(contribRole);
createdWeb.RoleAssignments.Add(contribRoleAssignment);

The above code will break the permission inheritance, so the created site will have unique permissions, this code could be refactored to also create Owners and Reader’s groups.