Are you speaking about a coven where people actually gather in physical space to hold rituals? Or are you speaking about "covens" such as the on-line ones here on SOM?
If you're talking about on-line covens then initiation isn't necessary. In fact it isn't really possible in the true sense of initiation.
If you're talking about a coven that meets in real life then whether initiation is necessary depends largely on what sort of coven it is. Some covens and Traditions require initiation, some do not. Unless you are specifically starting a coven in a Tradition which requires initiation whether to initiate members is entirely up to the group.