This is at 75, but I've never been use to so much invite attention before. I dropped party and put my flag up to switch to a new party for limbus once and got a /tell for exp, and my flag wasn't even up for more than 3-4 seconds.
In the lower levels I didn't always get invites up until about the 30s. Below the 30s, you still run into newb party leaders who don't understand what corsair or bard(let alone any other advanced job) really do. Beyond that though, around the garlaige citadel levels, you get more steady invites. I made my parties from 55-67 ish so I 'm not sure how the invite rates are in that range, but 67+ invites were very often and mostly if there is even one party in your exp level range then you are going to be invited to it, given your flag is up fast enough before the leader invites another cor or a brd.
[ffxisig]173144[/ffxisig] Social: AssassinsofDeus PLD:75 MNK:75 COR:75 Lacaan:Seraph
If you need a bard to get "good exp" in a merit party, you're the weakest link.