I find this question unusual, as I do not see how it is halachically relevant whether one informs people about an upcoming event via email, phone call, or by personal conversation. It is more a question of etiquette, which is of course very subjective and depends on the intended recipient.
In our Congregation, on the one hand, there are older people who would resent an email invitation in that it assumes that they are “online” and read their emails, while many do not. On the other hand, there are young people for whom it would be completely fine to communicate via email, and perhaps even preferable. Thus I suppose the correct answer to your question is “It Depends”.
Lastly, I wish to reference an earlier JVO answer that I provided on the topic of “unveilings”, and whether there is anything Jewish about the custom at all.
Given what I wrote there, it certainly is preferable if a minyan is available for the Hakamat Matzeva, and this might be accomplished better or worse via email depending on the target audience – "It Depends!"
Is it proper to send an email invitation to an unveiling? The short answer is yes. but it's worth looking at the question from two perspectives, that of who comes to an unveiling and as a issue of etiquette.
As has been pointed out in a number of JVO posts about unveilings, an unveiling is not a second funeral. In many ways it is an unusual ritual because even though it is very widely observed, there is no halakhic (Jewish legal) requirement to have one and therefore no formal rules about what needs to be done. All of which begs the question, what is so compelling about an unveiling and what does this tell us.?
Most unveilings in America are done about a year after someone has died. Being together at an unveiling is a very different experience from that of a funeral. At an unveiling the mourners are not in the thrall of an immediate loss and often there has been a winnowing of memory. The meaning of a loved one’s life is becoming clearer and there is a kind of sharing that is different from that at a funeral. An unveiling often has a sense of intimacy and a spirit of continuity, which is its own way of honoring the memory of the deceased.
Which brings us to who should be notified about an unveiling. If either the person who is being mourned or the mourners are active in congregational life than it is appropriate to notify the community at large, either by way of the synagogue’s bulletin or through one of the many kinds of current social media. Email is a very effective way reaching a more intimate circle of friends and family, but one should be wary of the many ways that an email can appear impersonal or even slapdash. If your family is in the habit of sending out notices about family events by way of a general email, then that would be appropriate. If not, out of respect to the person you will be honoring, you should send personally addressed emails. When in doubt always err to the more personal, especially for a ritual as solemn and intimate as an unveiling.Lastly there is the question of whether or not you need a minion. The ritual of an unveiling ideally includes kaddish and when at all possible, one should gather together a minion. Therefore, if one has a very small family, the notification needs to broad enough to try and assemble 10 adult Jews. In today’s world, email can be a particularly good way of doing this, but again, the more personal the email the more likely it is to speak to people about the importance of their role in helping round out the minion and their importance to you.