Someone from outside the team who is added to a members-only channel, by handle or with an invite link, now joins as a guest of the team with that channel. Guests are listed on the team's Members page, count toward the team's guest limit, and are invited with the same invitation as any other guest, which also waits on Your teams. A team on the free plan can invite guests too.
With the team selected, a guest sees their own channels and nothing else of its Team chat. Removing a guest from the last thing they are in removes them from the team.
Replying to a message request makes its sender a guest of your Personal with that conversation. A direct message stays open while the two of you share a team where both of you use Team chat; otherwise it becomes read-only for both of you, with Invite as a guest to carry on.