Special players and ringers
In addition to your orchestra's regular personnel, you will most likely need the occasional "special" player, like harp or saxophone or contrabassoon, that isn't part of your regular contingent. Perhaps that player will play on only one piece for only one concert in a given season. Ideally, you would have a list of players you can call on when needed, and they would come to every rehearsal for that concert for the portions of the rehearsal when they're needed. If so, you could hold them to the same attendance policy that applies to your regular members. Or, if the part is very small, they could come to just a few rehearsals and the concert, as long as this won't be too confusing to your regular musicians.
However, sometimes it's very difficult to get amateur players who can fill these parts (a contrabassoon costs at least $25,000, so it's not an instrument very many amateurs can afford to own). You might be able to snare a music student looking for experience, or you might have to set aside a small budget to pay a professional to sit in with you on these special instruments. It's likely that these players will be available for only one or two rehearsals and the concert (or unlikely that you can afford more than that, if you're paying them). Consequently, the need for special instruments should definitely be part of your conductor's programming considerations and your budgeting process. It's also a good idea for your board to talk over your ringer-hiring policy before the need arises. Where should you draw the line between a paid professional and a volunteer, and should you ever pay anyone, if all your regular members are unpaid?
With all of the above said, I don't like to condone the practice of hiring ringers in general. A very occasional ringer is okay, for a special instrument or in a last-minute emergency when a key player has to suddenly miss a concert. Filling out your orchestra's ranks with ringers regularly is not okay. First, it damages the group's social and musical cohesion. Ringers who only come in for a short time don't bond with the group members or form a meaningful commitment to the group. And, even if the ringers can jump in and play successfully with only a few rehearsals, their presence can still be musically confusing for your regular members who have gotten used to rehearsing without them.
Second, bringing in ringers is sort of insulting to your regular musicians. Your regular musicians show up every week and work hard to prepare themselves for the concert. By bringing in ringers at the last minute, particularly ringers doubling parts that your regular musicians are playing, you're effectively telling your own musicians that they aren't good enough and that these "better" musicians don't need to work as hard to maintain as strong a commitment to the group. This is unhealthy for morale and may encourage poor attendance among your regular musicians.
Finally, if you feel the need to bring in a large number of ringers for every concert, it's a sign that your group is in poor health. If your own musicians aren't good enough for the music you're trying to play, you need to be playing different music, or you need to adjust your expectations, or you need to recruit a different set of regular musicians who can perform at the level you expect. If you don't have enough musicians to play the music you want to play, you need to being playing different music, or you need to come up with a recruiting strategy that will fill out your ranks with committed regular members. Or, perhaps your lack of musicians is a sign that your group isn't fun to play in and that you need to make some serious changes. Regardless, you need to revisit your vision and goals and make some fundamental changes to your structure and policies.