Jacek, I am new to ofbiz, but perhaps what you are looking for is here and the lines below it in the method:
<call-service service-name="createPersonAndUserLogin"
in-map-name="personUserLoginContext">
<result-to-field result-name="partyId" map-name="tempMap"/>
<result-to-field field-name="createdUserLogin"
result-name="newUserLogin"/>
</call-service>
Calls the simple method createPersonAndUserLogin located in
PartySimpleMethods.xml
I hope this is what you are looking for.
-CJ
