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.
The owner will then have the option to approve or deny your request.
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 join a Lead, Contact, or Deal that you are able to view.
It's also only possible to add Collaborators that already have access to the Lead, Contact, or Deal in question. If you cannot be added as a Collaborator, please check your permission settings first!