Colleague turned up at a meeting uninvited.

[liquidsquidli]

Is this weird? it was a complaint resolution meeting I was chairing and colleague had directly contacted the complainant who told colleague when the meeting was. Turned up and told me they were coming to the meeting too.

they had no context or information or had even discussed with me the strategy.

how unacceptable is this?

It depends on the relevance of the complaint to them / their role etc. You've not given enough details for anyone to give a view otherwise. Did they attend as a companion for the complainant or in some other capacity?

Everyone is entitled to a witness/companion. Did you ask why they were there?

I agree with both PPs. You haven't given enough information and they are entitled to have someone with them.