Malaysia's status with respect to the Joint Investigation Team (JIT) is unclear. If Malaysia is not on the team then it isn't clear whether the reason is that they haven't been invited or that they haven't accepted the invitation.
Malaysia had an independent streak that predates both MH370 and MH17. For example, when the International Criminal Court did not investigate allegations of war crimes at Iraq the Malaysians formed their own tribunal. They tried in absentia George W. Bush, Tony Blair, Dick Cheney, Donald Rumsfeld and others. They concluded that the invasion of Iraq was unlawful and that war crimes were committed at Abu Ghraib prison. A year later they found Israel guilty of genocide of the Palestinian people. It is no wonder if the Malaysians find it difficult to get cooperation from countries like The Netherlands, Belgium, Ukraine and Australia who are friends with the US, UK and Israel.
When MH17 was shot down Malaysian officials negotiated with representatives of the Donetsk People's Republic (DPR) when the other countries wouldn't, resulting in the hand-over of the black boxes to the Malaysians. Surely the black boxes would serve as tickets admitting Malaysia into the JIT, but Malaysia merely handed them over to Dutch officials. I still wonder if that came as an unpleasant surprise to the DPR.
A six member Malaysian team accompanied the train and aircraft that transported human remains to Amsterdam. The bodies were examined by the Dutch DVI team, and the Malaysian DVI team had no opportunity to do likewise. The Malaysians didn't complain.
In August Malaysia's defense minister said he was confident that MH17 was shot down by a BUK missile and dismissed other scenarios. At about the same time (it may have been as early as late July) the JIT nations entered into an agreement to keep investigation details secret if any country objected to a release.
In early September Malaysia said they were seeking support from relevant countries and other parties for Malaysian investigation and security teams to return to the crash site. However, a few days later a DPR representative said the crash site was safe enough, that all Malaysia needed to do was ask, but Malaysia had not done so.
Russia's defense minister said that the Russians were ready to assist and support the Malaysian investigators. To date there has been no sign that Malaysian investigators have in fact received support from Russia's defense ministry.
On Nov. 05 the Malaysian prime minister said that Malaysia expected to play a major role in the investigation and that they were welcome by the Dutch to do so. He also said, "We have preliminary knowledge (of what took place), but it has to be backed by evidence". Malaysia apparently is not interested in evidence that does not fit their "preliminary knowledge".
On Nov. 19 Malaysia's inspector-general of police said that Malaysia must be made a full member of the Joint Investigation Team and not merely a participant in the investigation. Malaysia's ambassador to The Netherlands also expressed displeasure that they aren't members of the JIT.
If Malaysia has been treated badly all this time, then they've waited a very long to complain about it. MH17 did not suffer mechanical failure. The pilots cooperated with ATC and didn't stray into restricted air space. Malaysia has no reason to agree not to disclose any information that would be contrary to the interests of The Netherlands, Belgium, Ukraine or Australia. If they're not already a member of the JIT, then it is hard to imagine their getting in before their government signs the agreement.
The Malaysians have given many mixed signals. Maybe that's just due to their culture, and maybe it is the result of behind the scene political maneuvering.