its difficult if you dont have a mic, normally we will have a party going on the ps4 where people chat so you wont hear the conversation going if you join the fireteam. for myself anyway if i'm in the tower, out on patrol, doing normal strikes or crucible and there is free space feel free to join any time. psn messaging service is a bit rubbish but that can also be used if you're looking for help with something specific, or otherwise invite people to your fireteam