1.
You tell someone to come along to encourage them in a friendly way to do something, especially to attend something.
There’s a big press launch today and you’re most welcome to come along.
= come on
PHRASAL VERB : V P
2.
You say ‘come along’ to someone to encourage them to hurry up, usually when you are rather annoyed with them.
Come along, Osmond. No sense in your standing around.
= come on
CONVENTION
3.
When something or someone comes along , they occur or arrive by chance.
I waited a long time until a script came along that I thought was genuinely funny...
It was lucky you came along.
PHRASAL VERB : V P , V P
4.
If something is coming along , it is developing or making progress.
Pentagon spokesman Williams says those talks are coming along quite well...
How’s Ferguson coming along?
PHRASAL VERB : V P adv , V P