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Owner: Team owners have control over the team. This includes all functions such as adding and removing members and guests, assigning additional team owners, and changing team settings.
Members: Members are people within your organization who are invited by a team owner to join the team. By default, members can use all necessary features, but cannot change settings for the entire team.
Guests: Guests are people outside your organization who are invited by a team owner to join the team. Unfortunately, external users cannot become members, they are limited to the guest role. The extent to which the guest role is restricted depends on the team and channel settings. As a rule, however, the guest role is very limited.
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