Google Plus currently can be joined only via invitation. Actually its under Beta phase, So Google is testing its functioning before opening it for all.
Good news is that Google plus is going to open for all on 31/july/2011
According to Google Plus team, it can be a day or two sooner but not late.
So, from 31/july/2011 you do not need any invitation to join Google Plus.
If anyone need Google Plus invitation, leave a comment below with your gmail ID.