The fact is, any man can
sacrifice to the gods at any time, and chances are reasonably good that if he
is pure enough of heart the gods will accept his gift.
Bloting with a group is another matter. Normally a group will only
allow picked people onto a blotstead, because the presence of one false-hearted
person, even if his friends don't realize he is false-hearted, can ruin a
blot and anger the gods against everybody. The gods can see into our hearts,
and they always know.
About the only way to get into a group blot anywhere, I should think,
would be to get into a group somewhere and give people a chance
to get to know you.
Hope that helps.