The code to retrieve the members of a specific Team is quite easy, it's a standard QueryExpression, but contains a linked entity with a condition inside, it's good as start point for similar queries.
// Id of the specific Team
Guid teamId = new Guid("DAC1F09E-02EF-E311-A2AB-D89D67630DBC");
// main query returing users
QueryExpression userQuery = new QueryExpression("systemuser");
// take all columns
userQuery.ColumnSet = new ColumnSet(true);
// this is the intersect condition
LinkEntity teamLink = new LinkEntity("systemuser", "teammembership", "systemuserid", "systemuserid", JoinOperator.Inner);
// this is the condition to use the specific Team
ConditionExpression teamCondition = new ConditionExpression("teamid", ConditionOperator.Equal, teamId);
// add the condition to the intersect
teamLink.LinkCriteria.AddCondition(teamCondition);
// add the intersect to the query
userQuery.LinkEntities.Add(teamLink);
//get the results
EntityCollection retrievedUsers = service.RetrieveMultiple(userQuery);
// fetch the results
foreach (Entity user in retrievedUsers.Entities)
{
// Id of the user
var userId = user.Id;
// FullName of the user
var userFullName = user.Contains("fullname") ? user["fullname"].ToString() : "";
}