The short answer
No, as long as users are set up correctly. Each client lives in its own sub-account, and HighLevel keeps customer data logically separated by unique IDs. A client added as a user inside their sub-account sees only that sub-account. The risk sits with agency-level staff: an agency admin controls every sub-account, so staff who handle only some clients should be Account-type users.
How sub-accounts keep clients apart
HighLevel is multi-tenant software, and its security overview says customer data is logically separated using unique IDs, with authorization rules built into the architecture and validated continuously. For an agency, each client is a sub-account, which HighLevel also calls a location, with its own contacts, conversations, pipelines, calendars and phone numbers. HighLevel's guidance is to add a client as a user inside their own sub-account under Settings, pick a role, then choose which modules they can open, rather than giving them any agency-level access.
Agency staff are the users to watch
Agency users come in two types. An Agency-type user works at agency level, and an agency admin controls all sub-accounts, SaaS products and billing. An Account-type user can open only the sub-accounts assigned to them, which is the setup HighLevel suggests for account managers, sales teams and fulfillment staff who look after a defined group of clients. High-impact actions such as creating sub-accounts, managing users, opening billing settings or transferring a sub-account can be restricted separately, so a junior team member does not need admin rights for daily work.
Limiting access inside one client
Within a sub-account, users are either Admin, with full control of that location, or User, whose access follows granular permissions for areas such as contacts, conversations, payments, workflows and Voice AI agents. The Only Assigned Data toggle narrows a user to contacts assigned to them, opportunities they own and appointments or tasks linked to their name, which suits a sales rep who should not read colleagues' conversations. Dashboard export rights can be switched off as well. HighLevel's own support staff can get logged, time-limited access to an account for at most 24 hours per request.
More answers in HighLevel questions. For costs, see GoHighLevel pricing or run the cost calculator.