Players connot join the party while you are engaged in combat

[AquaRyu]

While mob is engaged we can't do many many things:

invite
Disband
Log off
Equip skill
Swich job
warp
...

[Tillman]

Obviously it's to prevent RMT...

[Felis]

While mob is engaged we can't do many many things:

invite
Disband
Log off
Equip skill
Swich job
warp
...

You can still equip skills during combat. You only need to put your weapon back for a moment.
If you are not in combat but your weapon is out, you can't equip skills, too