You can use the getMemberByID
method of MemberService
WS interface, whose XML request is the following:
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:urn="urn:api.ecm.opentext.com" xmlns:urn1="urn:MemberService.service.livelink.opentext.com">
<soapenv:Header>
<urn:OTAuthentication>
<!--Optional:-->
<urn:AuthenticationToken>?</urn:AuthenticationToken>
</urn:OTAuthentication>
</soapenv:Header>
<soapenv:Body>
<urn1:GetMemberById>
<urn1:memberID>?</urn1:memberID>
</urn1:GetMemberById>
</soapenv:Body>
</soapenv:Envelope>
In general MemberService (available at http://your-OTCS-server/cws/MemberService.svc) provides you a lot of other user-related functions:

You can easily import the WSDL into, say, applications like SoapUI and perform all the testing you may need.