If you want people to do something, you need more than a single message.
It doesn't matter how well you optimize the message, frequency trumps optimization.
A single announcement asking members to join the community, sign up for an event, write a blog post, participate in a discussion, or share their knowledge isn't going to be enough.
Instead, you need multiple triggers prompting members to do these things. You need to walk the fine line between gaining maximum participation and spamming members.
Too often we err on the side caution. Our fear of being labelled as a spammer by the few is stopping us from attracting the many. There are many communities that failed simply because the community manager feared being labelled as a spammer.
This doesn't mean you should repeatedly send the same messages. That's just annoying. It does mean you need one message and then to evolve the message. You can invite someone to participate in discussions, update people on the progress of that discussion, highlight the controversial viewpoints of that discussion.
We need to appreciate the danger of being perceived as a spammer, but not let it overwhelm our actions. We need to balance this with our need to make meaningful things happen on behalf of the community. That typically means you need to send more than one message.