A quick fix for server admins using teamspeak 3 when guests cant view who in channels:
To join a channel the i_channel_join_power of the server group (found in servergroup/channel/access) must me equal or higher than i_channel_needed_join_power of the channel (found in channelrights/channel/access). To view the members in channels, you have to do the same for i_channel_subscribe_power. Probably this is too low or not set for your members.