Please see the C4FM notes section on this page:
http://manuals.sharkrf.com/openspot2/en/homebrew-mmdvm-connector.html#c4fm-notes
What you can do with DTMF is the same as with a DMR transceiver - start group and private calls to the server. The server will then decide what to link/unlink for you. For more information on talkgroup linking/unlinking please see
http://manuals.sharkrf.com/openspot2/en/dmr-usage-notes.html
The OS2 will only transmit 1 call and monitor 1 server/room at a time. However, if the server supports it - like BrandMeister - then you can link multiple talkgroups which will be transmitted to your transceiver.