Look into CAE certificates (if you have them for your goats not worth the risk to bring any females that are un-tested), make sure you have a good lice, and also a mite treatment available (whatever you use normally), and if you are offering boarding you really should make sure that any on-coming goats are fully vaccinated against both clostridial and pasteurella disease, and you have an agreement ready to cover any emergency vet costs and what to do if goat dies while at yours....
Be prepared to deal with orf coming into your flock (not really a problem just something to be aware of), plus I guess there are other bio-security issues....
....
bit of a minefield having someone else's goats on your farm IMO...