If a client in Vinyl is synced from Karbon, you won't see the Add member button on that client's page. That's expected: Karbon-synced clients and their contacts are managed in Karbon, and Vinyl picks up changes from there.
This usually comes up when a meeting isn't showing under a client and you try to fix it by linking the attendee, as described in Tag a Meeting to a Client.
How to get the meeting to show under the client
Find the attendee's email. Open the meeting in Vinyl and note the email address of the client attendee. Vinyl matches meetings to clients by this email.
Update the contact in Karbon. Make sure that person exists as a contact in Karbon with the same email address, and that they're linked to the correct client organisation.
Wait for the sync. Changes in Karbon take some time to come through to Vinyl. See My Karbon sync is taking a long time if it's slow.
Check the client in Vinyl. Once the contact appears as a member of the client, open the client's page to see their meetings.
If the contact now appears under the client but the meeting still isn't listed, contact our support team via chat. Include:
The meeting title and date
The client name
The attendee's email address
If the contact isn't coming through from Karbon at all, see Contacts not appearing in Vinyl after Karbon sync.