Setting up a point-to-multipoint configuration is much like setting up a point-to-point link with an access point. The difference is that multiple clients connect to a single AP. You can use any sort of antenna, but you should generally choose one with the narrowest beam width that will cover the area you're interested in. That helps reject noise from all other directions, and minimizes the noise that your network will cause for other people.
You should pay particular attention to the "hidden node" problem when dealing with long-distance point-to-multipoint links. On a simple point-to-point shot, this isn't a problem, because both nodes can hear each other by definition. But suppose you put an access point on a high point and attach a high-gain omnidirectional antenna to it, thus allowing multiple clients to connect. It is very likely that some clients may not be able to hear the traffic of others, so transmission collisions will occur. An increase in traffic passing through your AP can bring down throughput considerably.
One common method for dealing with long-distance point-to-multipoint collisions is to sectorize your AP: that is, add multiple radios attached to tight beam sector (or other) antennas to your AP site. Setting each antenna to a different channel and making intelligent use of polarization tricks can help reduce the number of clients associated with each AP, which reduces the problem of collisions.
If your AP supports it, you can also use RTS/CTS (Request To Send/Clear to Send). Using RTS/CTS guarantees that clients do not transmit simultaneously?each frame must first be approved for transmission, then acknowledged by the access point. Unfortunately, there is considerable overhead involved with RTS/CTS, so it is usually left off by default. If your AP is in a prominent place and shows many receive errors, consider trying RTS/CTS. Run a throughput test while the network is under typical load without using RTS/CTS, then turn it on and try again. The error rate should go down considerably on your AP, but your throughput may suffer. Fine-tuning RTS/CTS can be difficult?since 802.11b gear has become cheaper, it is frequently more effective to simply add more equipment to accommodate more clients.