You should reference the Community Service https://github.com/OpenNTF/SocialSDK/blob/master/sdk/com.ibm.sbt.core/src/main/java/com/ibm/sbt/services/client/connections/communities/Community.java
This Method...
public boolean addMember(Member member) throws CommunityServiceException
{
return getService().addMember(getCommunityUuid(), member);
}
Examples are here. https://greenhouse.lotus.com/sbt/SBTPlayground.nsf/JavaScriptSnippets.xsp#snippet=Social_Communities_API_AddMember
These snippets should resolve your issues.
You can use this API...
You can get the Raw XML and process http://www-10.lotus.com/ldd/appdevwiki.nsf/xpDocViewer.xsp?lookupName=IBM+Connections+4.5+API+Documentation#action=openDocument&res_title=Creating_a_request_to_join_a_community_ic45&content=pdcontent