Your attendance policy
I feel strongly that all your orchestra's members should be expected to attend every rehearsal for a concert they will perform in. Sporadic attendance and a lackadaisical attitude toward attendance among your players damages both the musical product and the social cohesion of the group. Amateur players, especially, need consistency during rehearsals. It's easy for them to get confused when they aren't sitting next to the person they usually sit with, an important solo or the cues they rely on aren't being played, or the overall sound balance is off. Furthermore, your orchestra is meant to be fun, so people should feel like they're among a group of friends, not a group of strangers who pop in and out whenever they please. Your musicians should view rehearsals as a serious commitment.
However, people get sick and go on business trips and have family emergencies. They will inevitably have to miss rehearsal once in a while. 95% of your musicians will be fine most of the time. They will be committed and responsible about rehearsals and communicate expected absences to the personnel manager and their section leader. The other 5% are the flaky ones who unexpectedly miss rehearsal without warning, who can't follow through on their commitment to the group, or who simply disappear (yes, this actually happens). It is for these people that you should have an attendance policy. The policy won't necessarily encourage them to be more responsible, but it will give your personnel manager some guidelines to follow and will make any decisions seem less arbitrary if the player has to be asked to sit out a concert or to leave the group.
After a good bit of debate on the subject, the RCO decided on the following for our attendance policy: "Musicians are expected to attend all rehearsals. If a musician misses more than two for a given concert cycle, this warrants a discussion with the Personnel Manager and/or Music Director, and they can ask the musician to sit out the concert if they decide it's warranted." Since we generally have about eight rehearsals for each concert, missing two means missing 25%. We left the policy very flexible, however, to allow each case to be considered individually after a thoughtful discussion with the musician to understand their situation. We will have to see how this policy pans out in the future.
Don't wait too long to decide and implement your attendance policy. During your first year, you will likely have to be pretty forgiving, since your players will only be just beginning to work your orchestra into their regular schedules. However, if lax attendance takes hold at the beginning, it could be much more difficult to tighten it up later.
Note: You will most likely need the occasional "special" player, like harp or saxophone or contrabassoon, that isn't part of your regular contingent. Your regular attendance policy may or may not apply to these special players; either way, this subject deserves your considered attention as well.