8.0 Update: Still mostly works, see comments
This very small addon, along with the LFGlist addon, automates the tedium of creating and joining groups, especially useful for shared lockouts when mount farming.
It enables you to create a macro that will search for a custom group with specified keyword.

To use, download the addon and make a macro that says "/lfgjoin AAA"
Replace AAA with any keyword you want.

Use macro, and the LFG tool will pop up and you will see any groups with that keyword.
That's it!
This addon works in tandem with the LFGlist addon, which creates an LFG group with a specific keyword. LFGjoin could also be used in a non-mount farming environment with multiple macros of different keywords.

Here are two other useful macros (not related to this addon; remove quotation marks)
"/script ResetInstances();"
and
"/script SetLegacyRaidDifficultyID(#)"
Replace "#" with the number:
3 - 10 Normal
4 - 25 Normal
5 - 10 Heroic
6 - 25 Heroic

For those new to mount farming on alts, here's a link to Asmongold's guide

This addon is extremely lightweight. If you'd like to save a click try Premade Group Finder also on Curse.

Thanks to Ro/Gello of Underhill for the original code.

Also check out Leaderchange