You can manage who can collaborate on your Deal (as well as Contact or Lead) using the Collaborators box in the right-hand column of the screen.
Adding collaborators is extremely easy – just click the grey + icon in the box, and start typing a name.
If you have Full Data Access enabled under your Manage User Settings on Sell's Enterprise Plan, you'll see the option to Become a Collaborator – this won't require any further approval from the deal's (or contact's, or lead's) owner.
Aside from giving you a way to quickly check who works on a given project, you can also filter Contacts, Deals, or Leads by using the Collaborators filter, available in the filters column.
Note: You can only be added as a Collaborator to a Lead, Contact, or Deal if you have permission to view that record. If you cannot be added as a Collaborator, please reach out to your Admin and check your permission settings first!