Having promoted your group in many places increases your chances of attracting unwanted company trying to join your group... People not on that sailing, people trying to market products, people who might want to learn about when someone might be out of town, people looking for a 'Disney dating site', etc... As such, you might find it important to screen your group membership.
The easiest way is to use the Facebook feature of asking up to three questions of anyone applying. Here's a link showing you how to do this, as well as some suggested questions.
How to set up screening questions on Facebook
Having them answer questions can help make the vetting process pretty easy... if someone doesn't answer them, or provide reasonable answers, you can simply Decline.
If you don't want to base your decision on just them answering a question, there are several things you can look for in a potential member profile which can put you at ease or raise warning flags, but any one of them might not be either a deal breaker or sealer:
Positive indicators
Disney-related likes, groups, photos, etc...
Family photos and members
FB member for a long time
FB URL which includes their name (or a reasonable facsimile thereof)
Warning signs
Very short time on Facebook
Membership in LOTS of groups (200+)
Business-focused pages (Only Followers, not Friends, etc...)
Facebook profile URL which does NOT include their name (or a reasonable facsimile thereof)
Gender mismatches with photos (someone updated HIS profile photo with a picture of a pretty girl)
Another way to vet Add Requests at times is to ask your cruise group! It's easy to see common last names, but not everyone has those in a party sailing together, so if you run across a locked-down profile ask your group if they know the person!
But if they answer the question/s satisfactorily, their profile passes the initial sniff test or shows warning signs, and no one in the group knows them, many group admins directly ask questions of potential members via Private Message before allowing them to join. Below are some examples you are free to use, the key being to use ones you are comfortable using but will likely be known to people on that cruise. Ask the group for other suggestions as to how they vet members... That's why they're here... to learn from and help others!
"Thanks for requesting to join our group. We ask that anyone who joins our group be booked on this particular cruise. In order to confirm this please answer the following question/s:
1. What is your stateroom number?
2. What is your stateroom category?
3 What is our first/last/3rd/etc... port of call?
4. What port are you looking most forward to?
5. What port will we be in on Day X (always more fun if Day X is actually a Sea Day!)
6. Which restaurant are you looking more forward to?
7. Are you booked on this cruise?
8. When did you book it? NOTE: If their answer is BEFORE it was available to book, that tells you something...
If you requested to join our group in error, please let me know and I will delete your request. Thanks for understanding!"
Remember that our goal as a cruise group admin is to help our members enjoy their cruises above and beyond just booking a cabin on a DCL ship, and drama in your group doesn't help with that! Taking a few moments to help keep a group 'safe' from the types of people mentioned above by screening your members can help, particularly if you promote your group far and wide!