Add wrapper for group join API (#633)
Add wrapper for group join API (#633)

Needed for new "joinable" groups.
krombel/matrix-js-sdk,krombel/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,krombel/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk