If the person is already a Dreamler user, you can invite them by opening the project details from the project list.
Once you have the project details open click on Share Project to open the project participant menu.
Invite project participants by adding their email address and click on the Invite. You can add multiple people by separating their email addresses with comma. One you have invited users you can manage user access through the same dialog. Read more here: Manage Project Participants.
Persons that does not already have a Dreamler account will receive an email with a registration link. Those who already are users will see the invite after logging in to the client.