Spend the 50 certs required to get the AMS utility and then be sure to equip it on your Sundy before deploying one. Once equipped and bumbling around in one, press B to deploy as a spawn point. Note that you can only deploy a Sundy if there isn't another one in the immediate area.
You can buy the ability by going to Certs > Vehicles > Sunderer > Mobile Spawn for 50. Once you've done that, go to your Vehicle tab when you pres Esc, select Utility I think it is, and select the AMS.
you will see circles on the minimap of other sunderers too so you know where you can not deploy yours