'too_many_write_operations' on 'sharingAddFolderMember'

This 'too_many_write_operations' error indicates "lock contention". That's a result of how Dropbox works on the backend. This is a technical inability to make a modification in the account or shared folder at the time of the API call. This error indicates that there was simultaneous activity in the account or shared folder preventing your app from making the state-modifying call (e.g., adding, editing, moving, copying, sharing, or deleting files/folders) it is attempting. The simultaneous activity could be coming from your app itself, or elsewhere, e.g., from the user's desktop client. It can come from the same user, or another member of a shared folder. You can find more information about this in the Performance Guide. (That's written more for uploading files, but it has useful context.)

In short, to avoid this error, you should avoid making multiple concurrent state modifications. That won't guarantee that you won't run in to this error though, as contention can still come from other sources, so you may want to implement some automatic retrying, following the 'Retry-After' header, if given.

Also, note that the sharingShareFolder job itself can runs asynchronously. If you get an async job ID back, you should use sharingCheckShareJobStatus to check the status and wait until it's complete before issuing more changes.