Go to the home page of your or someone else's community, follow the link "invite to the community and look at the top of the page is a button that says "Find and invite people". You will see a list of your friends and if you want to invite the community of all of them, check the "Select all".
If you want to invite individuals, select the tick only those users whom you would like to see in the community. If a lot of friends, scroll the list until you reach the end and select all friends for invitation. To invite the selected people in the community, click "Send invitation." Repeat this step for each page of the friends list.
In addition to people that are in your friends list, you can invite the community and other users – for this you need to choose the option "Find and invite people". By searching for keywords or other parameters to find the right people and the right of the user profile, click on the link "Call to community".
Look for new users to send invitations can be arbitrarily long and in any quantity – but unlike the friends list, the invitations in this case can be sent individually to each user.
If the first page of search results you are not satisfied, click "More" to access further results. Also you can invite to community their classmates, fellow students or work colleagues, selecting the appropriate search filters.