How to declare if yopu don't know the element ammount?

Question

HI,

 

Ive got the following question?

how can i declare an array if you don't know how manny elements the array will finally contain ? 

 

here is my example.  I sed the wcfUser array could contain 1000 elements ( simply because i need to do a test to add 1000 users) but the wcfUser array could at the end just contain for example 15 elements ( you don't know it yet).

 

here is my code;

WcfUser[] usersInGroup = new WcfUser[1000];

// Get users GroupUsers request
var request = new GetGroupUsersRequest
{
requestId = _requestId,
serviceProviderId = "DO_onepro",
jSessionId = _jsessionId,
sessionId = _sessionId,
domain = "1pro.deanone.nl",
groupId = "1PRO_DEMOIVO",
requestUsername = "mhussein"
};

// GroupUsers response
using (var client = new TechnicServiceClient())
{
var response = client.GetGroupUsers(request);
var errorCode = response.errorCode;
var errorMessage = response.errorMessage;
usersInGroup = response.users;
}