Adding users is easy. Plus, all plans include unlimited users so you're welcome to invite everyone on your team.
Simply share the sign up link https://www.insycle.com/getstarted/. After they fill it out and login to Insycle they'll join your plan automatically.
Insycle plans are tied to the underlying app account, that's how Insycle associate users to the right plan. For example, for Salesforce, all users that belong to the same org ID are part of the same plan, for HubSpot it works the same way based on the portal ID, etc.
To connect more apps to Insycle check out this article.