Getting people set up with a Mattermost account is something that happens when deploying and configuring the Mattermost workspace. A Mattermost system admin can provision Mattermost users using one or more of the following methods:
By default, team admins can invite people, including guests, to a Mattermost team, and all users can add existing Mattermost users to a Mattermost team or channel, unless the system admin has restricted the ability for you to do so.
Inviting people to a team sends an email prompting recipients to create a Mattermost account on your Mattermost workspace.
Adding an existing user to a team or to a channel makes those users team or channel members.
Add users to a channel by selecting the channel name and selecting Add Members.
Guests are restricted to only the channels you select.