Send has a function where overlapping recipients in other groups can be removed upon sending. This is known as deduping.
To use this function you will first need to select two groups (or more) that share a number of contacts. For example, an invitation list, and then a group containing all those who have already RSVPd.
Then deselect one of the groups. All contacts in this group will be removed from the recipient total, even if they overlap in other selected groups. Per the above example, this would leave you with all invitees who have not yet RSVPd.
When a Campaign is sent out immediately, the figure specified next to Current recipients shows how many contacts will be receiving the Campaign.
Please note that when you schedule a Campaign and have selected both groups, the figure will return to the total of both groups before it was deduped.