I had a hard time trying to figure out a way to retrieve members from a marketing segment in my code.

The link-entity between contact and msdyncrm_segment isn’t materialized in CRM. I even raised it here :Basic operations on segments using the Segmentation API

Solution:

Finally fetch xml came to rescue. You can use below xml which can be executed by WebAPI or Organization service and it works on both static and dynamics segments.

<fetch version="1.0" output-format="xml-platform" mapping="logical" returntotalrecordcount="true" page="1" count="5000" no-lock="false">
<entity name="contact">
<attribute name="fullname"/>
<attribute name="contactid"/>
<order attribute="fullname" descending="false"/>
<link-entity name="msdyncrm_segment" from="msdyncrm_segmentid" to="msdyncrm_segmentmemberid" alias="bb">
<filter type="and">
<condition attribute="msdyncrm_segmentid" operator="eq" uitype="msdyncrm_segment" value="bfc9d5d6-d6aa-e911-a859-000d3a3159df"/>
</filter>
</link-entity>
</entity>
</fetch>

Now lets try to execute this first in web api.
Use below format : https://orgurl/api/data/v9.1/contacts?fetchXml=<xml here>

Now lets try to execute this using organization service, note that i am using paging here because the number of records can be more than 5000:

var fetch = @"<fetch version='1.0'  page='##pagenumber##' count='5000' no-lock='true'>
<entity name='contact'>
<attribute name='fullname'/>
<attribute name='contactid'/>
<order attribute='fullname' descending='false'/>
<link-entity name='msdyncrm_segment' from='msdyncrm_segmentid' to='msdyncrm_segmentmemberid' alias='bb'>
<filter type='and'>
<condition attribute='msdyncrm_segmentid' operator='eq' uitype='msdyncrm_segment' value='6f5846d5-e5ae-e911-a84b-000d3a802d92'/>
</filter>
</link-entity>
</entity>
</fetch>";



 var contactss = organizationService.RetrieveMultiple(new FetchExpression(fetch));                  
                        foreach(var contact in contactss.Entities)
                        {
                            var id = contact.ToEntityReference();
                            Console.WriteLine(contact.GetAttributeValue<string>("contactid"));
                        }

result:

Apart from this fetch xml there is no way to retrieve members from the segment.  There are some issues and limitation i faced during this operation, will try to share them in another post.

I hope this helps!