First, add the Plus.API:
mGoogleApiClient = new GoogleApiClient.Builder(this).addApi(Drive.API).addApi(Plus.API).addScope(Drive.SCOPE_APPFOLDER).addConnectionCallbacks(this).addOnConnectionFailedListener(this).build();
Then you can switch accounts like this:
public void onClick(View view) {
if (view.getId() == R.id.sign_out_button) {
if (mGoogleApiClient.isConnected()) {
Plus.AccountApi.clearDefaultAccount(mGoogleApiClient);
mGoogleApiClient.disconnect();
mGoogleApiClient.connect();
}
}
}
For more, see here.