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I usually ask, or if we have been running dungeons for a bit I will just send the invite with a comment that it's "Just in case you are interested, no pressure. I just had fun running with you."
I don't spam invitations. You never know who you will get that way, and in my guild we are trying to maintain our current culture, if that makes sense?
When the game first started I was getting unsolicited invites a lot. But it doesn't happen much anymore. I think it's probably common for a guild to send one to you or ask after you run around with them.
I always ask. I personally find it extraodinarily rude and distasteful to spam invites, and i hate people who do that. Alway ask. It's simple common courtesy.