Are you trying to grow your community membership?
(Before you say "duh...everyone wants a larger community, Rosemary," let me say that a very small, focused, engaged group can be just as valuable in some cases as a million-person fandom.)
OK, that's out of the way.
You want to get more members.
Add Value First
First, make sure your community is magnetic, so that it is automatically built to attract people who will find it useful, entertaining, or beneficial to them.
Don't Be Afraid to Ask
When's the last time you told your current community members that one of your goals is to grow the community? What's that you say? Never?
If you've built it right, the members of your community will feel some degree of ownership, and will want to help achieve the goals. After all, your goals are their goals, right?
When you ask them to help bring in new members, explain why. Point out how a broader community will make their lives better, increase the resources available, or help your organization achieve its larger mission. Heck, maybe you just want a more diverse set of opinions in the community!
Bottom line--you won't get help until you ask for it.
Empower the Inviters
In Crowdstack, the ability to invite other members is a permission, so you can open it up to the entire community, or you can lock it down.
Perhaps you want to make the ability to invite others an exclusive privilege for those who have been in the community for a while, or perhaps you want every single member to be bringing a "plus-one."
Incentivize Bringing in New Members
In a recent software update, we added a new item to Crowdstack's Activity Points Formula--"Member Invitation Accepted". Now, you can bump up the weighted value of that action, giving members who successfully invite new members a points bonus.
Let your membership know how you're incentivizing member recruitment, and consider making it a community-wide goal to reach a new member milestone.
Make it Simple
It's already pretty easy for your members to invite their friends; there is a default widget at the top of the home page with an "invite" link or button. All they need is an email address to send the default invitation.
You can also choose to provide your members with customized invitation messages they can insert in the email, or community graphics they can use to draw members in from their other websites or blogs.
Create a featured topic that includes some suggested wording, and/or a graphic they can use when promoting your site. You could even offer suggested wording for social posts to promote the community.