The distinction between covenant membership and particular election is explicitly stated in the following words:
But that the general election [generalis electio] of a people is not always firm and stedfast, a reason readily offers itself: because to those with whom God makes a covenant God does not invariably (Hoekema's trans. of protinus) give the spirit of regeneration by virtue of which they would persevere in the covenant even to the end; but the outward change [externa mutatio] without the interior efficacy of grace which might have availed to keep them is a kind of middle way [medium quiddam] between the rejection of mankind and the election of a small number of the godly.
Here Calvin clearly teaches that the adoption of people into the covenant of grace does not mean that each covenant member will invariably be saved. Rather he calls the covenant here a kind of middle way (medium quiddam) between the rejection of mankind and the election of some. You could say that covenant membership is here pictured as a circle wider than particular election, but narrower than mankind as a whole.
[A A Hoekema, 'The Covenant of Grace in Calvin's Teaching', CTJ 2/2 (1967)]