The plain or polite volitional form of a verb, followed by the particle ka, is used when expressing a suggestion in the form of a question: "Shall I...?" or "Shall we...?" Similar to this construction but more polite is the negative polite question masen ka.
The particle demo in a sentence involving a suggestion indicates a thing offered up as a suggestion, but only as one of a number of possibilites. It replaces the direct-object marker o.
Kōhī de mo nomō ka.
Shall we have coffee or something?
Ame ga futte iru kara takushī de ikimashō ka.
Shall we go by taxi, since it's raining?
Watashi-tachi no sensei o pātī ni shōtai shimasen ka.
wouldn't you [like to] invite our teacher to the party?
Hope this helps!
°˖ ✧◝(○ ヮ ○)◜✧˖ °こゆうきあいはら°˖ ✧◝(○ ヮ ○)◜✧˖ °