I'd say it's definitely worth considering getting a mic. as it's not only useful for co-ordinating but also adds to the fun! That said - I've yet to meet anyone from DoD who has an issue with anyone jumping on a Fireteam - after all, if someone does want to play alone (maybe levelling a new character through the story missions) then they can always set the Fireteam to invite only. Jump on - kick ass - have fun!