If you'd like to add a large group of users, Onboarding is well suited for the job. This can be accessed under your Community Tools / Project Tools, and involves creating an Opportunity (landing page) and attaching one or more Sources to it (e.g. Community Filter, Email List). Users added through onboarding will remain in that Opportunity Pool until they activate ("Convert") their account (e.g. click the link in the email or the project on their Community Homepage). The Opportunity Pool will also allow you to send reminders to all unconverted users.
If you're looking to invite one or more users ad-hoc, just click the person icon to the left of the search bar in the upper right-hand cover of your screen, from anywhere within your Project. This brings up the Quick Invite overlay, which allows you to rapidly invite up to 100 users to specific Project Teams or directly to any existing Opportunity. Quick invite sends a very brief email (which you can customize).