How often may a player make a weak opening - 8 or 9 pts before it is deemed to be part of their system? When convention cards are not used at a club should this type of opening be alerted?
It is legal to agree to open at the one-level on a hand that is 8 points or greater and there's no requirement to alert it if it's a natural bid. With one partner, we tend to keep our bids up to expected strength but we have an agreement that our third-seat opening might be lighter than otherwise expected for an opening hand and we sometimes pre-alert this as a courtesy. If a pair is regularly opening light in first or second seat without compensatory distribution, they're going to get themselves into a lot of bad scores, even though they might pick up the occasional good score by making it harder for an opponent to enter the auction.
Does this pair have discovery methods to determine whether partner has opened light? Opening light in third-seat has the benefit of being able to pass partner's response, since, unless it's actually a forcing bid, we wouldn't consider it to be forcing, given that partner is a passed hand.
Regularly opening at the one-level with 7 points or fewer would constitute an illegal agreement but 8 or 9 points is legal.
Bear in mind that it's not so long ago that it was common to pass with 13 or even 14 points hand if there was nothing of interest in the hand. Modern bidding makes it rather uncommon to pass a 12-point hand. Expectations change with the times and so we fall back on the Laws when needed and they offer a legal minimum of 8 for a one-level suit opening.
If the partnership frequently open otherwise unremarkable 8 or 9 counts this should be disclosed to the opposition. ("Frequently" just means sufficiently often that partner comes to expect it, so that it's no longer a surprise to partner. That doesn't have to be very frequent.) As Tag says, there's no requirement to alert, but the Blue Book says this should be disclosed on the convention card (3D1). If they don't have convention cards, as a minimum requirement they should explain it to their opponents at the start of each round.
If this is causing trouble at the club, I would suggest the committee make a local regulation that pairs playing such methods have to bring convention cards and present them to their opponents. Everyone playing standard methods can continue without convention cards, but if this pair fail to comply they can be had up for playing an unlicensed method, or failing to disclose their methods adequately.
One should note 3A1, which already states, "Pairs are required to have two fully completed system cards...". It applies to all, not just those who play systems which can be unpleasant to face, although many clubs ignore the regulation.
I suggest that you ask them to pre-alert, i.e. tell opponents before the round begins.
I think it's accepted that not all club games can insist on convention cards, as it puts off novice players and scratch partnerships. We have written into our club rules that pairs that play complicated or unfamiliar systems must present their opponents with properly completed convention cards, and that anyone without a convention card can expect to be ruled against in case of doubt about their methods.
EBU regulations are for EBU events primarily. While it is normal for clubs to follow EBU alerting, announcing and permitted agreement regulations, they certainly do not have to follow other regulations and it is rare for any club to insist on 3A1. As VixTD says, I believe clubs should insist on system cards for unusual systems.
But this thread seems to be ignoring:
7 A 3 Strength of Opening One-level Bids
A one-level opening bid in a suit, whether forcing or not, must by agreement show 8+ HCP and, in first and second position, follow the Rule of 18. Natural 1NT opening bids must show 9+ HCP.
As I said, clubs generally follow EBU regulations for permitted agreements so if this pair is regularly opening 8 and 9 counts that do not follow the Rule of 18 they are playing illegal agreements in first and second seat. If they do follow the Rule of 18 that just means they are playing Acol light openings as played by a vast number of pairs in this country from the 1950s to date and I see no reason to alert this. Of course you get local practices and if players in a club use heavier openings than is normal in England except for one pair I would have thought people should just get used to it: they will find light openings the moment they play elsewhere.