1. join a group/team
Professional
Social
glad to have someone on board
'To get on board' with something means to accept and join a project or team or to contribute to it. It has a positive connotation of cooperation and support. This can be used in social contexts ("I told the tidy towns committee I was on board to help them"), but you are more likely to hear it being used in relation to talking about specific projects or proposals: "We hope to have a new consultant on board soon to help us with these issues."
2. agree with, be willing to participate in something
Professional
Social
get on board with the plan
To 'get on board' with something means to agree to an idea or a plan. It implies that there was some doubt that the person in question might accept the project or proposal to begin with. It has a positive connotation of cooperation and support. This can be used in social contexts ("We can't convince our oldest daughter to get on board with moving house"), but you are more likely to hear it being used in relation to talking about specific projects or proposals.