If the architect/designer is on staff, or handles all of your projects...
Set them up as a builder-side user, so they have the ability to add and edit specs or selection items, as well as interact directly with the client. When an admin user on your team adds them to the system, the admin can specify whether the architect can have access to some or all projects, as well as controlling their ability to update the Schedule, update templates, and view financial information.
Depending on the specifics of your situation, you may decide to limit that architect to just certain projects or to block that person from seeing financial data. Your account admin can control this by going to the Contacts > My Team.
If the architect/designer has been hired by the client...
And are acting on behalf of the client, it is usually helpful for them to be in the communication loop just like your clients. So, it is often best for them to have the same type of login as your clients. This gives them the ability to ask and reply to questions, view and make comments on selections, have access to pricing and change orders, post messages, view and attach files and photos, and view the Schedule (if you've shared it).
To go this route, simply add the architect or designer just like you do additional clients from the Project Setup page. Go to the client section and click the link to add another one. If your plan has a limit on the number of clients on a given job, and you don't see that link, that means you've already maxed out the client limit for that project.
You could instead add the architect as a trade partner, but that would only allow them a limited view of the Specs & Selections and would not allow them to access to the Question or Messages pages. The architect could participate in comments on selection items, but only for those items where you explicitly grant permission to do that. If those things are not important, then the Trade Partner List may be the way to go.