>> But I've no idea how to assign User Task to Group/Role. <<
In the Task properties window, expand "More properties" and add GroupId to the tasks DataInputSet property value, the in the Assignments property value set the group name by doing an association: GroupId=XYZ where XYZ is the name of your group.
>> Why I get the different bpmn file from the web-based designer and the eclipse-based designer? <<
The small differences are because Eclipse tooling uses the internal jBPM parser where as Designer uses the eclipse.bpmn2 API. The differences are very small, for example <bpmn2:process .../> instead of <process ..../> and the geneartion of node IDs is different (Designer currently does not generate very human-readable ids but we are working on that for future versions). Both tools generate valid BPMN2 that follows the specification, and is executeable in the jBPM runtime.
Hope this helps.
Glad to help. Would you mind putting this thread as answered if you have no further questions on it? Thanks.