If you run a closed community, you should either encourage members to invite as many friends as possible, or restrict them to one invite per month.
The two strategies are very different.
The first requires the most work. You have to keep finding ways to stimulate members to invite friends. If it works, you get a long period of sharp growth followed by a period of levelling off.
The second is easier. Giving members just one invite increases the value of the invites. Members will headhunt the best people they can find. Or they might run competitions with the invites as prizes. You get a stronger community that aims for a growth rate of 100% per month.
It pains me when new web companies get the two confused. Kwippy, Naymz and BrightKite are all guilty of giving members up to 15 invites. You can’t go half-way on this, either unlimited invites, or just one.