Community Moderators (and all members, when the Moderator chooses the "anyone can invite" checkbox) can invite people by visiting the "Invite," accessed on the web at the top navigation when you click "Invite." On narrower screens, it may be hidden underneath the "..." menu at the top of the screen. On Mobile, it is under the left navigation menu, accessed by clicking the button at the top left of the screen, then clicking Invite.
Once there, you have three options for inviting members: You can copy and paste a link that can be used by anyone to join the community. It can be pasted in email newsletters, written on a wall, or texted to anyone. Your members can forward these to others, as well.
Or, you can invite people by pasting their email addresses in the "email" field. This will send a single-use link to anyone who's email you paste in the box. The links will expire after a single use, and provide a clearer curation boundary around your community.
If you are inviting people who will be joining as Moderators, you can select the checkbox to make it so that when these users join they will automatically be Moderators.
Lastly, you can upload a .csv file with hundreds or even thousands of email addresses.
You can take a moment to edit the invitation people receive on this screen as well.
It might also be a good time to visit your community Settings page to edit the Welcome Message that your members receive.