Hi @MartinH,
What version of Mattermost are you using? We recently updated the login flow in v3.0, so I would recommend upgrading if you haven't already.
Can you clarify what you mean by wanting users in one start group?
If you mean you want users in one team, I would recommend you set up one team and then send people invites to that team. You can do this from the Main Menu, using either "Invite New Member" to send an email invitation or "Get Team Invite Link" to get a link that will take users to the sign up page for your team.