Having been on both sides of the agency-client relationship, I recognize the temptation to sometimes sell around the technical team. Maybe you've had bad experiences with a client's technical leadership or ran into the "no" team. You might get the sale keeping IT out of the process, but you are likely to fail in maximizing the benefits to your company and, more importantly, the customers. Let me walk you through the potential pitfalls of this approach and then suggest a better way to proceed.
If the engagement requires sharing data and integrating systems, as most do these days, then it is better to learn about any technical mismatches before the contract is signed. Every advance in standards to make things work together better seems to be offset by the growing complexity of solutions. Closed ecosystems favored by Apple create simplicity by enforcing homogeneity, but most corporate IT is a gumbo that requires highly skilled technicians to navigate. Get IT on your team early to help ensure success later.
Another advantage to engaging the technical team is to gauge their ability to execute. This is less about judging their skills than seeing their attitude towards the project and learning what else is on their plates (more on this next). It could be that key technical skills, tools or processes are missing and by knowing this beforehand, you can address the resulting risk during your planning. Obviously, this needs to be handled delicately to not offend anyone or cause people to hide these issues. This should be straightforward for the savvy agency professional used to navigating complex engagements.
My final reason for including the technical team from the beginning is to identify where the work you need done fits in their project backlog. Smaller companies often have huge project backlogs, while larger companies might have complex governance processes designed to prioritize projects. In either case, your engagement might be DOA if the client's technical team can't do their part in a timely manner. But if they are invested in the effort and understand the business benefit, then you might be amazed at how quickly any project backlog difficulties can be resolved.
How do you avoid going down any of these paths? Embrace the geeks early on in the process and insist they are part of as much of it as possible. By gaining understanding of the business side and how your agency plans on doing its part, the technology team will be able to spot technical mismatches and see where this effort fits in their project backlog. And by getting to know them in these meetings, you will have a better idea of their ability to execute. Most importantly, they will have a vested interest in your success rather than looking at your engagement as a distraction from whatever they are trying to accomplish.