>> 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 tooling support valid BPMN2 that follows the specification, and is executeable in the jBPM runtime.
Hope this helps.