To invite people to your account hover over "People" in the sidebar and click on the "+" icon.
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This will take you to the invite people page:
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From here you can add the email addresses of each person you want to invite. This can be 1 person or many at a time. Just make sure to add 1 email address per line in the input.
Next you can choose a role for the person or people you are inviting. When you invite people they will be given the same role, group and projects so if you want different settings for people you'll need to invite them separately. See more about roles and permissions here: Roles & Permissions
Then you can optionally add the person to 1 or more groups. Adding people to groups isn't necessary but it is a great way to categorise people. See more about groups here: Groups
Finally, you can optionally add people to projects. This isn't necessary but it's an easy way to add people you invite to many projects so when they first login they will be able to see the projects you want them to see.
When you're happy you can click on "Save" and the person or people will be invited. They will receive an invite email with a link to join your account:
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When they click this link they will be taken to your account where they can specify their name and password.
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Once they have done this they will be taken directly into your account so they can see the projects you have invited them to.
They will then be able to login again at any time using their email and the password they set.
You will also be able to edit this persons role or archive them at any time to change or restrict their access to your account. See more about this here: People