Adding teammates to a Channel
Invite flow, pending vs. active members.
Open the Channel and add teammates from your workspace — this is separate from inviting someone to the workspace itself (see Inviting teammates), since a person needs to already be a workspace member before they can be added to a specific Channel.

Pending vs. active
Adding an existing workspace member to a Channel takes effect immediately — no acceptance step needed, since they're already part of the workspace. This is different from a first-time workspace invite, which does require the invitee to accept before they show up as an active member anywhere.
Removing someone from a Channel
Removing a teammate from a Channel revokes their access to meetings in it going forward — it doesn't remove them from the workspace itself, and doesn't retroactively hide anything they may have already seen or exported.
Someone can't see a meeting they should have access to
Confirm they're actually a member of the Channel the meeting lives in — see Channel permission levels for what different membership levels can and can't do within a Channel.