There are a couple of ways you can request to add seats to your Sendoso account.
The first is to contact your CSM directly to discuss adding licenses to your account. They will have all of the information regarding pricing and process.
The second way to request more seats is to do so through your account!
First, navigate to your Team Settings page from the menu on your dashboard.
From there, click Users at the top. You should be able to see at the top a banner that contains how many seats your plan has and how many you have left over to use. Additionally, there will be a link to Request more seats.
Once you click that link, you'll be directed to open a request with your CSM.