If you receive an invitation to participate, it is because the brand has specifically chosen you to join its campaign.
By clicking on the invitation, you can read the brief, the number of publications required, the elements required and the offer for your participation.
If you wish to accept the invitation and join the campaign, click "Participate".
You can check the status of each publication in the campaign's task list inside Collaborations, just click the task and it will take you to where you should take action.
To find out more about how to draft a proposal, please read this article.