A Member is a User who has access to your Workspace. A Member of a workspace can optionally be made an Owner which gives them full control over the Workspace including managing other Owners.
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The Free plan allows for adding an unlimited number of Owners to a Workspace. Inviting a non-owner to your workspace is not supported on the Free plan.
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When you want to add a member to your workspace you create an Invite. For security reasons an invite expires after 7 days. When you create an Invite on the Free plan and it is accepted the User is automatically added as an Owner to the Workspace.