Team Members are most likely your colleagues who send out contracts out on behalf of your company. You can only be an active member of one team at a time. If a team member is deactivated (for example in case the person is leaving the company) they will no longer have access to their account.
Note: When a team member is added to the team, they are automatically put on the same plan as the team owner.
Team Viewers (guests) could be for instance lawyers or investors who need access to view contracts but will not be sending contracts on behalf of your company. The team member or team owner will not have access to team viewers' documents. A team viewer can only be directly removed from the team.