yes you can participate in someone else's guildleve, but thats not on your demand, thats by theirs.
The point is to make other people need you in their group, not make it a voluntary effort "if they feel like inviting you". There must be a mutual interest for both the one lfg and the one lfm to look for each other. XI had this through the party size being established at 6 people, so you were always looking for 5 people to join you.

What do you suggest SE do to improve the lack of repeatable content?
Make participating in the current repeatable content easier and more universal. Make parties look for members, make members look for parties, improve their tools to do so, and remove aspects of the game that are in direct conflict with this concept- monster grinding, first and foremost. It needs to go, for everyone except those who want to solo/duo. They can not divide their playerbase between two different activities that are both used for the same purpose, as well as require group effort to work efficiently. They need to be able to control our progression to make sure that everything works as intended. If any group size is fine, any group set-up is fine, and you can progress in various ways that the developers can not control, we come to the current situation that plagues the game. XI suffered from this, but not as much, because the developers controlled the party sizes and set-ups. Yet they didn't control what we killed and where, and the end result was crabs, crabs, crawlers, more crabs, manaburns, monkburns and arrowburns.