Your best bet would be to first retrieve the list of members within the channel using the Web API method channels.info.
Each channel
object will contain a members
field containing a collection of user IDs -- those who have joined the channel.
You would then have two options depending on your preferences:
1) Use users.list to retrieve a list of all team members (including each user's presence) and narrow list of users down to those listed in the members
field from above.
2) or, you could look up each user from that members
field, one at a time using users.info.